Ask HN: Is the fintech Blockchain excitement deeply flawed?

Context: If you follow startup trends you probably heard fintech sector is getting more excited by the day with blockchain, altho not necessarily Bitcoin itself. They highlight, among others:

- possibility of smart contracts - fully transparent databases - peer to peer network - trust(less) service layer between - efficiency savings

Mostly created the list from this podcast: https://soundcloud.com/a16z/business-blockchain - a really good episode btw.

The question: is the fintech ignoring the most important factor, which is the ever increasing competing mining computer power applied to BTC network being the reason it's so safe only because mining is rewarding. If at any point an ill intentioned player controls over 50% of computer power would the whole system would crumble? Doesn't that make it unpractical for any startup to implement a network they can actually make safe?