This assumes that the increases in computing power were not added via a mining pool, in which case the additional computing power is making things worse.
This is one reason that many view mining pools as one of the worst failures of bitcoin / other blockchains.
It's not total centralization, but it's a far cry from the level of decentralization that would be possible if mining pools did not exist.
There is a lot about this topic in the new, free book from Princeton authors titled "Bitcoin and Cryptocurrency Technologies" (a great read)[1]. There is much research being done on the front of finding the best way to eliminate mining pools or at least stop them from potentially centralizing consensus to just a handful of people.
[1]: https://freedom-to-tinker.com/blog/randomwalker/the-princeto...
51% being honest is fundamental assumption of a cryptocurrency [1]. The assumption gets stronger as total computation power increases.
[1] Its necessery but not sufficient condition to make the whole state dishonest. You would need to have >50% computation work done.