I've said it before in numerous threads, but I fully agree with you Barry.
I mean I love sonic 3 and knuckles, but when it comes down to it, it comes mostly from S3k's magnificent presentation and specific parts around the midst and the end of the game. When judging S3k as a whole experience, maybe half if not more of it exists of the same things that have been ruining games of todays generation(not just sonic_. I'd almost say perhaps s3k may be one of the first games to do that. Which is something people call style over substance/depth. The game no doubt has some excellent levels in it, excellently executed level gimmicks, but mostly you are just "rushing" through the levels without thinking too much. The latter is really a problem because it removes focus, it removes your full involvment. It doesn't require you to fully pay attention and do your best. It mostly goes like: get hit, land on a halt, spindash and pickup the next few rings so it doesn't ever matter how many times you get hit.
In sonic 1, apart from green hill, no level let you play like that. I'd say yes it's partially because of the absence of the spindash. If you got hit? You had to slowly accelerate yourself to the next few rings, which were placed a lot more scarcely in sonic 1. Not only that, but the level design does not encourage mindlessly running and jumping. It's cleverly designed where you're constantly paying full attention. Whether it's platforming, walls, bumpers or spings, spikes. There's a lot more threatening hazards in sonic 1, and I love that about it. It keeps you playing and makes that whole playthrough the more satisfying. I know from myself when I finish sonic 1, I feel more fulfilled. And I wish newer games would take more inspiration from sonic 1's real concept of "speed is a reward from playing well"
I think the only cons are 2 things. The game isn't that long(kind of hoping of an additional level in the new version), and the anticlimactic final boss. Which the other 2 games do a lot better and I think it is because of those two things the other games will always be considered the better games.
Let's talk hypothetically if taxman and co made a new sonic game. I'd love them to take inspiration from sonic 1 as well. Seriously screw the people that don't like being slowed down and think for a moment. A new sonic game should bring best of both worlds imo