That can be said to HTML. Some old pages break in new browsers.
But, IMO, MS does a pretty good job on BC, so, you shouldn't be worried.
But is MS going to care about the Mac SL plugin 5 years from now? Personally I couldn't care less, but this is something that I am sure was a factor in many companies' decision whether to use SL or not.
BTW, you can open a 15 year old html page in any modern browser and will probably work just fine. Check out this 14 year old web page. Works fine except for a few broken links.