The way I use Strava sounds very much like what the Mileeater Diary is for. But automated, sharable and with better analysis tools.
Like all web services, I don't trust Strava to exist next year. It'll go bust, suffer a tragic data loss or get borged by $EvilCo. So I have copies of the raw GPX files on my own sever.
There won't be any problem reading GPX files in 15 years. Even if there isn't any software that does it, it's a clear human-readable format, and if necessary I could write my own. The files themselves will be fine as long as they're kept on a running system rather than gathering dust on some media somewhere.
Even AUK ride records from before 2000 are not available on the current computer system, as they are on paper somewhere and it would be an impossibly enormous task to scan it all.
That's an argument *for* electronic records, not against it.