....yes I know that. The question was "would they", not "why did they". And yes, yes they will abandon devices soon after launch.
Yup, I got the Lumia 800 on a 2 year contract and a few months later the 810 came out with Windows 8 and my phone was stuck with 7.5, eventually 7.8.
But let's also keep in mind of the situation of the time where there was Microsoft making the OS and Nokia making the Hardware. Everything is under one roof these days (for the most part) and learning from past mistakes is something MS seems to be in a position to be doing recently.
Also with the news spreading about W10 working on the Raspberry Pi2 (and for free) it seems MS is doing their best to make W10 as compatible and optimized as possible.
Whether they are using W10 (PC) or W10 (Mobile) I don't know.... But if they are using the PC version, then one can assume the mobile version will be even more flexible with lower system specs.