Is a 51% attack on bitcoin likely?
No.

Bitcoin is an economical system. To become a pool operator, you must have 1) large amount of bitcoin or 2) large amount of hardware. Both requires you to be an "investor". In contrast to Proof of Stake, it is not easy to sell everything and leave Bitcoin in short time, which means you're informally "contracted" to Bitcoin. To put it another way, the only proper way to profit is long term mining. That requires you to ensure that it won't decrease in value. (1% increase is actually a lot for them) As a consequence, you can't attack Bitcoin even if you have the power to do that, as it'll affect its value.

This was the reason Emin Gün Sirer's Majority is Not Enough: Bitcoin Mining is Vulnerable was controversial.