I think a 10% of world broad money supply for crypto as a whole would be a reasonable long term target.
That would basically put crypto at about half the total Euro broad money supply.
I would then assign Bitcoin maybe 2.5% of that total crypto amount, putting BTC total market cap at about 125 billion. Oh, but we're already there with Bitcoin! That's one of the fundamental reasons why I say it's overbought. Syscoin, on the other hand, would need to grow by 718 times to reach its 125 billion "market share", STEEM by 455 times, BitShares by 345 times, Cardano by 125 times, for example. I estimate 50 viable survivors 3-5 years out, with most dominating their own market niches and carrying a fairly relative value - if they're all viable, then their value should be very similar.
Look at how XRP, ETH, DASH, LTC, etc. are doing; they're much closer to their eventual "pieces of the 5 trillion crypto cake". Ethereum is already a third of the way there!
We might be on the verge of crypto's first "rolling correction" where the high flyers correct and the oversold move higher, narrowing the spread between the two.
That's how I treat the big numbers - 5 trillion is a lot - and I don't think BTC will have more than a 2.5% market share in 3-5 years. It's already down to 55%, and that's after a major rally to new highs that pulled it off its all time lows below 38% just a few short months ago.