They don't need someone better, just someone different.
Whatever way you dress it up, whatever he did last season, being 1 point above relegation at the end of February is diabolical.
You could cut him some slack if they were just having a shit season and were sat around 13th-14th without looking like they'd finish any higher but not likely to go down, but take out the nostalgia of "he won us the title" away, then if you don't sack a manager for this, what do you sack them for?
Him leading the team to those heights only highlights by how much he's lost them this year. It's not like they're playing good football and getting unlucky. They're shit and it doesn't look he's anywhere near turning it around.
And I'm going to guess if he gets them relegated and kept his job, he's not going to show them the same loyalty by sticking around to manager a Championship club for 5 years if he has a better offer elsewhere. This is business.