Kit vs RTR - may I throw in my $0.02 (US)?
This is a dilemma I have faced many times over the 40 odd years I've been in this hobby. For me it comes down to what I want to do. Some parts of this hobby are much more enjoyable than others. For instance, I enjoy hand laying track - for a short distance - like say a diorama. But over a whole layout, for me that's just a chore. Especially when flex track really looks so good!. (Keep in mind, I started this hobby when fibre ties (sleepers) was all there was.)
I've built a few locomotive kits over the years and I'm very proud of them. But when I see a RTR loco that looks good AND runs well - for me there's no choice.
I know this is not what the OP meant - but it just brought all this to mind.
But regarding the marketability of old kits - I've bought a few over time and very much enjoy working on them - often using them as a basis for a fun kitbash, replacing old rough castings with newer versions.