Classic Sonic Isn't Past Modern Sonic
First off, this is just what I think from what I've played.
I've played most of the Sonic games available on Steam and watched cutscenes and playthroughs of games not available on Steam, but to me it's very obvious that Classic Sonic and Modern Sonic are just 2 different dimensions, but the amount of people I have seen defend the fact that it's just the past and there's only a timeline split after Generations is crazy especially since the games say it's another dimension (Sonic Forces: https://youtu.be/qrs8UozGEDo?t=286).
I have even seen someone say that it can't be a different dimension because they only say it in Sonic Forces and they personally don't like the game so it's not canon, so the Tails line isn't canon either (real reason I have seen someone use).
Here is the evidence I have found so far.
Age:
• Modern Sonic is 15 in Sonic Heroes (Sonic Heroes Manual Page 6: https://archive.org/details/sonic-heroes-usa/page/n3/mode/2up).
• Modern Sonic has his birthday in Sonic Generations making him 16 AFTER Generations.
• Classic Sonic is 16 at least after Sonic 3 (Sonic Jam Character Profiles: https://youtu.be/5a3HfbhMYdE?si=YP6jcjb\_N98QxuUo&t=213).
• Modern Sonic only thinks Classic Sonic is "another Sonic" not him from the past (Sonic Generations and Sonic X Shadow Generations: https://youtu.be/FeaLm0HAEvo?t=365).
Green Hill:
• The very start of Sonic Generations takes place before the Time Eater starts the events of the game, showing Classic Sonic in Green Hill. Meaning the Green Hill in Sonic Generations is from the Classic World and not the Modern World.
• In the original Sonic Generations even when Classic Sonic was still past Modern Sonic, Modern Sonic only thinks Green Hill was familiar. Not his Green Hill (Sonic Generations: https://youtu.be/QMopx-E4la0?t=515).
• In Sonic X Shadow Generations Modern Sonic says Green Hill looks different. (He could be referencing the colour being drained and them not being on South Island, as Tails says this right after (Sonic X Shadow Generations: https://youtu.be/FeaLm0HAEvo?t=177).
Planet:
• I think the Classic Sonic Games are set on another planet like Mobius, with Eggman travelling there from Earth and the Modern Sonic Games are set on an Earth that has elements of Mobius.
• Similar places exist in both worlds. Like both worlds having Green Hill but slightly different Green Hill's.
• Similar events could happen in both worlds like Eggman tricking Knuckles into fighting Sonic happens in Sonic 3 and before Sonic Adventure (Sonic Adventure: https://youtu.be/ucUJKGU7tl0?t=870).
• Sonic 3 is not how Modern Sonic and Modern Knuckles met, but Modern Eggman tricking Modern Knuckles into fighting Modern Sonic could have been how they met.
Meeting Eggman:
• Modern Eggman would know who Modern Sonic is because he is famous (Sonic Adventure 2: https://youtu.be/\_9rIND08eHM?si=f9b9WfPA1vTzp9jQ&t=3265).
• This cutscene doesn't seem like 2 enemies that have fought many times before it feels more like Modern Eggman meeting the famous hero Sonic and introducing himself for the first time and Modern Sonic is making jokes to tails about him looking like an egg (Sonic Adventure https://youtu.be/ucUJKGU7tl0?t=358).
Sonic's Reaction to the Chaos Emeralds:
• In the Classic World, Classic Sonic has seen and used the chaos emeralds multiple times.
• In the Modern Sonic World The chaos emeralds are very publicly known about.
• Public enough for the news to be talking about them (Sonic Adventure 2: https://youtu.be/\_9rIND08eHM?t=3258).
• Public enough for the government to send a spy to collect them (Sonic Adventure 2: https://youtu.be/\_9rIND08eHM?t=5464).
• This cutscene doesn't feel like sonic seeing an emerald he has already seen many times before. Instead it feels like him seeing a world famous chaos emerald for the first time (Sonic Adventure 2 https://youtu.be/ucUJKGU7tl0?t=313).
Amy:
• Modern Amy can't spin dash originally but learns how (Sonic Advance: https://sonic.fandom.com/wiki/Spin_Dash#Sonic_Advance Sonic Frontiers https://youtu.be/Dhdz\_8zXAOQ?t=682).
• Classic Amy can spin dash (Sonic Origins https://youtu.be/Dhdz_8zXAOQ?t=709 Sonic Superstars https://youtu.be/Dhdz\_8zXAOQ?t=814).
EDIT: TailsTube and Takashi Iizuka
• In the TailsTube clip u/Naija_Boi replied with, Classic Eggman Calls Modern Eggman "Eggman from the future" but Modern Eggman calls Classic Eggman "Eggman of the past" not younger Eggman.
• Takashi Iizuka stated "The Classic series began with Sonic 1 in 1991, and Sonic Adventure marked the switch to the Modern series in 1998. So in terms of the Sonic timeline, Sonic Superstars occurs after Sonic Mania and before Sonic Adventure. Whenever we develop a new game, we always make sure the story fits with the lore."
• Modern Sonics "present time" and Classic Sonics "present time" do not have to be the same time. Classic Sonic and Classic Eggman can be 16 and like 40 respectively in like the 80s, while in the modern dimension Modern Sonic and Modern Eggman can be 16 and like 40 respectively in the future.
• This theory works with Modern Eggman calling Classic Eggman "Eggman of the past" and also the timeline placement stated by Takashi Iizuka
(It's like calling G1 Optimus Prime a younger version of Transformers Animated Optimus Prime, or 10 year old ben 10 from the original show a younger version of the 10 year old ben 10 from the reboot, or kinda like 10 year old Bart Simpson earlier in the simpsons with gameboys and ataris is a younger version of the current Bart in the simpsons with smartphones and gaming PCs, the "present time" is earlier.)
Probably more out there but this is just what I've noticed from how much of Sonic I've played or watched so far.
If there's anything that proves them to be past and future feel free to let me know and Ill see if I can fit it into my theory and explain why they aren't. Just supposed to be friendly discussion about the timeline because lots of people are very set on Classic and Modern being the same, and I want to see peoples opinions.
Also it's a fun harmless theory if you want to show some evidence that disproves my theory that I can try to work around like u/Naija_Boi showed evidence that's great! That's exactly what I asked. But if you come in and just bring others down like u/Shaddy_the_guy by stating 100% fact and not showing evidence from the start and not having friendly conversation about a theory I had then just don't bother.