Nobody is playing 20 year-old save games.
That's not true. I still fire up my original version of TTD from time to time for nostalgic reasons. Virtual machines are a thing of beauty. Being able to retrieve those games from years ago and trying again (and again) to build it better can be fun, too. Especially when you have to deal with all those limitations that OpenTTD has made obsolete. Admittedly, I haven't done it in awhile, but the games are kept on my not-cold file server because they're definitely not cold data. However...
If they want to do so, they can open up any old version of OpenTTD and continue to play it.
Exactly. And I think OpenTTD has reached the point where backward compatibility can now be honorably retired. We can have the best of both worlds with what we have now. Old-school is preserved well enough that historical "data" can be accessed and...played. That gamer who's had the Civilization game running for literally years and years I think exemplifies the idea behind this ability to preserve and play forever, and I think that's a good thing.
Redirect made a comment about how difficult it is to get things into trunk - that patches are often used instead. I've always assumed that OpenTTD became tightly regulated to ensure the most basic aspects of the game are strictly preserved. There are certain aspects of the game that are cornerstones: the quasi-isometric/3D-from-2D layout, the "curved track is for Locomotion" limitation, etc. If you change it too much, it's not what I call a pure reincarnation of TT. It becomes something else. And that's where we're at right now. "Stifling further developments and improvement" to me is just another way of saying that v1.9 should be the last version whose intent is to strictly conform to TT's spirit. I feel like we're reaching the limit of what can be done without working beyond that.
In no way is this me being critical of anyone's work or anyone's ideas of what OpenTTD is, should be, or might become. It is simply an observation from someone who - if development stopped forever tomorrow morning - would still be left with something he will enjoy the rest of his life and be grateful for those who made it possible. I think that's the finest compliment you could ever give to any group of people who donate so much and receive so little in return.