It most assuredly is an issue! If you want to play most games in the store the lack of a full GB if RAM will impact you negatively.
And merely stating Win 10 will fix everything simply underscores how obsolete the 635 will be as Win 10 is at least 10 mos away!
I've owned the 635 from release and the build materials are cheap (crappy screen, plasticky back) but the build quality is high. This phone can survive a drop or two most definitely.
Unlike the 830s there are no touch issues. All the buttons are responsive as well as the screen.
It's the crappy and incomplete OS that is the problem.
Just as the Denim update brings features only to the newer phones you can expect the 635 to receive squat from Win 10.
Further reading this post I come to the conclusion that who cares if this phone becomes outdated did you spend 600 bucks on it ? I know I paid 125 for mine and by the time windows 10 phone releases there will likely be the 640/645 to drop another 124-150 on. By then all the issues with the 535 will be ironed out and you could probably grab it for around 75 bucks.
And yes as we start to see more windows phone 8.1 apps we will see a large improvement in ram management because there is no longer alocated ram on a per app basis.
So if windows 10 sees even further improved ram management on the back end we most definitely see it even on the lowly 520