When you start seeing these blocks >95% full consistently (a few weeks from now at present rates), expect a market reaction. 

If we see a price increase of >20%, expect the blocks to fill up.

This will slow but not stop the price rise. 

If non-dust transactions quadruple (in a few months and present growth rates), expect a permanent and growing backlog. 

Of course we will see this happening and so it will never happen. People will just migrate to other coins until one of them gets enough market share to suck off most of the new money and Bitcoin will be a collector's coin. A hobby coin forever.  It'll still grow in value,  but at a much slower rate than the new Cryptocoin, the one where wallstreeters get to be early adopters. 

I imagine a coin where blockreward gets reduced by 10% a year instead of dramatic halvings. Blocksize scaling is built in at predictable intervals.  Mining is done on the the more secure SHA512 or something more quantum computer resistant. It may have a full Turing-compliant scripting language for smart contracts, etc.