This topic has gone in a few different directions. I'm going to give general thoughts based purely on my personal experience.
I've come much further than I ever thought I would. If I was stuck forever with what I had a few years ago, I'd be happy with it.
I don't think most people will hit their genetic limit of size or strength because they're not willing to work hard enough, eat enough, or use certain compounds / keep increasing dosages / etc. That's a cold, hard reality of this sport, and I direct everyone who disagrees to Big A's famous The Truth thread -
https://www.professionalmuscle.com/forums/index.php?threads/so-sick-of-this-heres-the-truth.53353/. There's also how much you're willing to prioritize bodybuilding in your life, doing it as a single guy vs. married with kids, your robustness in terms of having low side effects and being able to handle a lot of strain on the body, etc.
I'm thrilled with where I am right now and this year I actually surpassed a long-term goal I set 7-8 years ago - 230lbs. at single digit body fat. Currently 224lbs. at 6%.
I've been making progress for 10 years with no signs of it slowing any time soon, and I'm 33.
The first time I ever dieted I ended up 166lbs. at maybe 9-10%. That's where I started. I spent a few years trying to figure it all out in the beginning, then a few years powerlifting, and now a few years of bodybuilding. I remember the first time I hit 190lbs. with abs. Not ripped, just a good outline of abs. Over the years I got better at training and dieting and got real lean for the first time at 196lbs. at 7.5%. I kept refining my training / nutrition / supplementation, and fast forward a few years to today - just had a Dexa scan of 220lbs. at 6% on Friday.
I've never had a mentor in this. I literally don't have any social acquaintances who have anything to do with fitness. I've never had a training partner. I've figured everything out on my own through reading magazines, Bodybuilding.com, boards like Professional Muscle, and other resources. And most importantly, TONS of trial and error. I've recorded everything I do for years - diet, workout, supplements, general notes on performance and how I feel, etc. - which is a key factor in how I keep progressing. But more than anything, I keep progressing because I want it that bad.
At 33, I'm the biggest, leanest, and strongest I've ever been. I made the best gains of my life, both in size and strength, over the last year.
Are there hard genetic limits? Most likely. I don't know that we'll ever see a stage-ready 400lb. bodybuilder, but we do have pros like Morgan Aste who are in pretty good condition at 360lbs. People wouldn't have believed that was possible 30 years ago.
I believe most limits are self-imposed - the mind is incredibly powerful. Think about Dorian Yates, one of the first to hit 300lbs. Once he did, others saw it was possible, and we started seeing more guys hit that size.
I never thought I'd be 190lbs. ripped, but I worked for it and got there. I said ok, I did that, so I can probably do a little more. And I achieved a little more and kept aiming for even more. 2 years ago I peaked at 241lbs. in the offseason; last year I peaked at 259lbs.; this year it's 275lbs+.
Most are capable of more than they realize, but never take the steps to get there.