I agree that miner rewards is a potential problem in the future. If transaction fees don't go up the blockchain will be vulnerable. I like the Idea for compressing the blockchain be getting old UTXO's to be used but I still don't think this would be the right way to go.

I think a better idea would be instead of halving the block subsidy every 4 years it should flatline at 1.5625BTC. This would insure miners will keep getting rewards and keep securing the network regardless if transaction fees increase. This level of inflation should be negligible. only 82,125BTC will be mined each year which is only a 0.39% yearly inflation rate.