I personally appreciate the DLC with 50 new levels.
It is understandable that gamers are not enthusiastic to see main features being only
available for Steam customers - but it is just tactics similar to Microsoft which Valve
do know well (we know where you came from, guys).
So it is clear to tie developers using their way of 'workshop' and try to spoil
any effort to make it fully available for other gaming platforms.
But this is caused by Steam, not the many developers who do suffer from that disease.
Concerning Train Valley 2 next to the inclusion of Ruby DLC (i.e. the 50 new levels)
the game engine Unity got a minor update (see the changelog on GOG: https://www.gog.com/forum/train_valley/changelog/post4
So for anyone not wanting the Steam way of DRM and excluding gamers,
not buying on Steam but buying additional content on GOG to show that they care
may be a better startegy than arguing against developers who need to earn money.
Steam just get everything for free (cf. "Money for Nothing") - by making gamers unfree.
"A client to rule them all ...". It is not unheard of - is it?
And gamers should take care that GOG does not follow that path, too ...
which is currently the case - getting more and more visible.
The GNU/Linux offering is larger on Steam than on GOG - it was the opposite before.