Do yall really think they would never remake Adventure 1/2 when the Crash and Spyro trilogies have been so successful? SEGA may not be the smartest company but it doesnt make sense for them financially to ignore such a huge possible money cow.
The main problem is that unlike spyro and Crash, Sonic games have aged pretty bad (In my personal opinion of course), so would it be enough to just put a fresh coat of paint on the game and sell it as is? or are you going to make deep structural changes to the game itself? I can't see any world in which they remove that alternate gameplay styles, especially considering how short the games would be if they were just the sonic story.
They'd have to change up the gameplay for sure. And in that, IMO, lies the issue.
They struck gold with the Generations/Boost gameplay for the 3D Sonic. Yes it's resource intensive (well, only if they want it to, Colours was a Wii game and had the basic boost formula down pretty well) and it needs decent level design (something that Forces, from what I've seen sorely lacks) but it works well enough. Generations wasn't popular just because it was a compilation game, it was popular because it was actually damn fun. The game made getting a better score in any given level a pretty fun challenge. You had a series of games (Unleashed - Colours - Generations - Forces) in which the regular Sonic gameplay became solid, stable and fun (for the most part).
The Adventure gameplay, while functional and acceptable at the time, aged poorly. A mere 4-5 years after the launch of the 1st entry (98/99) it felt super dated when it transitioned to the non-Dreamcast consoles (and the ports themselves were messy which didn't help). Heroes, Shadow, '06 all tried to keep it going, each with their own spin on the formula and none actually made it better in any way or form. Sure Heroes was pretty decent, but even that had it's own set of issues.
The closest I feel they've gotten to a proper Adventure gameplay style in modern days has been with Lost World. That game's abilities and movement for Sonic felt like they would work wonderfully in more open ended levels (not that Adventure 1 or 2 were that open in that regard) or a hub world without having to really change anything.
To keep this short, my point is that while they can take loads of inspiration from the Adventure Era in terms of style, feel or even having multiple characters, they absolutely can't keep that gameplay without improving it massively. However the issue lies in the fact that they themselves never managed that with the following entries before switching to the Boost formula.
At this point I think it's best they keep on the Boost formula or try something new entirely (and sure, bring on more characters, no problems with that), going back to something they never managed to make work is asking for pointless trouble IMO.