I agree that no quality or endgame upgrades for trains is an odd choice, but I guess that's what the mods are for.
That said, I was more than happy to build the base back from scratch in space age, and I find the expansion to be every bit as fun as the base game. So I endorse it. Especially as you already know how to do some things quicker.
But this wasn't the case — Space Age isn't only new content, but a complete re-balance of the original game. It is far far less grindy and requires much less baby-sitting of production.
Your first space platform, the ships, and the planets, are best thought of as unique Factorio-inspired puzzles. Each planet is like a Factorio game-mode to solve, with its own restrictions to design around.
I think those who have hundreds of hours in the base-game have to un-learn of the base game to pick up the DLC. Many of the complaints are pointed at the tech tree changes — they wanted an expansion on what was there, not a recreation. But for me now, I wouldn't recommend Factorio without Space Age.
All in all, the developers have a very different vision of what makes the game fun than I do, and that meant the expansion wasn't much fun for me. If I play the game more in the future, I'll probably do so with the expansion disabled.
“Good job! Halfway through the workday on Thursday, some big brain engineer in a distant department has decided to change the order of for loop clauses in the interpreter, so now it’s “for variable declarations; variable modifications; conditional checks {}”. They adamantly refuse to revert the change because “it makes more sense to group variable stuff together”. Prod is down now. Have fun!
There should be five options for each piece of tooling, none of which quite work the same way, all of which have a fan-base singing their praises, and three of which have critical problems you probably won't find out about until you try them. Then the one you pick gets abandoned when you're part way done. (this feature only for the Javascript and Python DLC)
Example farms should use old versions of libraries that are no longer maintained.
Interfacing with other farms should require manually faking a downgrade of some protocol you're using, because the older one is no longer available for you and they can't/won't upgrade.
You should be forced to design certain parts of the system in ways that are harder to use and constantly break when changing other parts, just so the appearance of those parts is "on brand". This should require re-writing functionality that would otherwise be supplied "for free" by a library.
When you're finally getting things how you want them, your farm should get cancelled and the whole thing abruptly burned to the ground because your parent company just decided to buy a different, existing farm instead.
This is funny because I already get the feeling a lot with management sim / automation type games that I'm pretty close to doing the kind of thing I'd do at work, except only the fun parts and without getting paid. Often that's the reason I quit playing these types of games after a while - having to deal with migrating legacy code after breaking API changes would bring this feeling to a new level I bet.