Wow, I have almost too much to say on the subject, but I'll try to keep it semi-brief
First, we really weren't able to show off the features we're most excited about when we released the newsletter. That being the new in-game screen as well as the new player models and improved stadiums. If you'd seen the updated 3d graphics and the new in-game environment, I don't think you'd still describe it as a 'tweak'!
There have been major changes there and the in-game screen and the 3d look awesome this year. Everyone that's seen it so far has been blown away by it.

We didn't want to show it while it was still being worked on and just give you a mockup or something. We really want you to see it in all its glory, and we're hoping to do that soon. If you play out any games in 3d, I'm pretty confident you'll seriously consider buying the game for that alone, even if the other features feel underwhelming to you.
On rosters, saying "rosters can be updated in OOTP18..." massively undersells what you get in an OOTP roster update. Sure you can do that in OOTP18 if you mean that you can spend a few hours moving MLB players around in OOTP18 and updating a few ratings. If that's all we did each year, I agree it wouldn't be much of a selling point. It's not though. That may be what you get in some other companies roster updates, but not in ours.
Every year we add thousands of new players to the roster set who have signed with MLB teams. Hundreds of them you've never even heard of before unless you work for a Major League team or something, and maybe not even then.
Do you know Jhosmer Alvarez or Rolfy Vargas? We do! Would they be in a OOTP18 roster mod? They're in OOTP19. Probably no one would notice if we didn't include them. But we try our best to get to everyone, no matter how obscure, because we really care about you having the most value for your money and by far the best roster update possible. We also add and update thousands of real draft prospects for the next 4-5 years of drafts in-game. Stats and updates for Indy Leagues and international leagues? The new amateur free agent signings in the KBO or ***? All in the update.
There's no way any fan created mod for OOTP18 is going to have a fraction of what we include in OOTP's rosters. Even just creating a basic bare-bones roster update for OOTP18 is probably going to take you 10-15 hours. If that's the case, why not just buy OOTP19 (at a cost of like $2-3 per hour saved) and spend that time actually playing the game or doing something else you enjoy?
Now the roster update might not matter to some of you, and if not that's totally understandable! Maybe you just play fictional leagues. But if it does matter to you, it's a very good reason to buy OOTP19.
Another thing is that we improve tons of stuff each year that we don't really mention, or that gets compressed into one or two sentences in the previews.
Improved AI for example, might not sound like much on its own, or it might just sound like fluff. But we put tons of work into improving the ai each year. Matt in particular does so much there. And while we don't have the time or space to detail all the improvements made, you can be confident they are there. If you doubt it, just load up a copy of OOTP12 and then OOTP18 and look at all the ways 18 plays better and is improved. So it's true that incremental changes are not sexy, but they do add up over time. Buying the game each year helps us to put even more time and effort into doing that, and then the incremental changes can become less incremental and more impactful year to year.
Speaking of which, there is the matter of supporting us with your purchase as some others have mentioned. Now that can certainly be a lame reason, and if we were releasing a crummy new version just to get the cash, I'd agree it wouldn't even be worth mentioning. But we've put as much work as ever into the new version of the game this year. Probably more
It's always really tough to decide what new features to include and what has to wait for another year. So even if the features you're most hoping to see didn't show up this year, we do need the support from folks buying the game to have the resources to add in even more features next year. And the features next year might be just what you're looking for.
So if everything seems still seems underwhelming to you, we're not saying you should buy the game just to support us and for no other reason. We don't want you to buy a product you'll be unhappy with or disappointed in just to "support" us! But if you're on the fence, and you just aren't sure what to do, that might be enough to tip the scale slightly in the direction of buying the game. So I mention it for what it's worth.
So is that enough to get you to invest in the game? Only you can answer that. Speaking for myself but also Markus, Andreas, Matt, Jorin, Daniel, Chris and everyone else at OOTP Developments, we really hope the answer is in the affirmative!