51% attack would be pointless unless those miners are willing to spend their energy without mining BTC. That's a horrible decision to do for them. It had never happen because these miners won't commit to plan on doing it. And I guess this will be the reason why it will remain decentralize even when a pool has the capacity to do it.
Unless, bitcoin hashrate drops in the future if the price doesn't go up and many miners quit. In that case, it would become easier and cheaper to commit a 51% attack. Also, there has already been such attacks on some altcoins such as bitcoin gold or ethereum classic. This means that if things fall into place it might be possible in the future, but at least at the moment if bitcoin continues like it does now, it shouldn't be a problem.