How many will listen to tape and vinyl in the next century?

After 100 years the main problem will be to find high quality original pressings. Most reissues are average sounding and they may not be able to carry the game of analog forward. We need seriously good sounding vinyls in plenty numbers in order to keep the interest going. 

Even today if we had to live off only the reissues a lot of us would lose interest in analog playback IMO

And thats the worry, the shortage of good LPs.