Having 55% of hash doesn't mean you'll make profit by attempting a doublespend. I don't see how you could gain more from one and dumping the assets price + your reputation as a pool than by just behaving and passively collecting your fees forever. Nobody event talks about how 1 or 2 pools control 70% of zcash hash (although that's probably because nobody uses it) and even BTC has 2-3 companies (not even proper pools) controlling the majority of the hashrate.
It sounds scary but there really isn't that much a non-malicious, profit-driven pool could even do. Even a malicious pool could at worst mine empty blocks for a while.