I can understand the logic behind sacking managers if you're convinced you have a better option lined up and have a better plan short term and long term to succeed. I honestly don't see how sacking a manager is always better for the club.
Take a look at Leicester, won the title, got to CL then their struggles were on the cards just like we did in CL under Moyes. So they go ahead and sack the man who got them their cos they wanted to look like some "big shot" ambitious club. Who did they go for? Oh yeah Craig Shakspear! The man they thought would be better cos he knows the club. They gave him money to spend and still they are lying in bottom 3. One of their big signings, Adrien Silva is in a limbo and I'm sure their fans would blame Shakspear for that too. Now they have sacked the manager who they gave a permanent deal in June to again look like "ambitious". Can someone tell me how sacking a manager like Ranieri and replacing him with Shakspear was ambitious? And their fans will be happy to see the club sack the manager. Cos that's what modern fans want. Just sack the bloody idiot and we'll be alright. Who will they now get in? Allardyce maybe? And how far can anyone else take them? Maximum 11th or 10th I bet. Is that ambitious enough for them? Or do they think they automatically deserve to be among the top 8 cos they won the title by some heavenly intervention a couple of years ago? If they just wanna avoid relegation and be around 10th then the same feat they can achieve by going on a 3 or 4 match unbeaten streak and pick up some form eventually. But no, modern fans want to call for the head without digging in with the team in tough times.
Going by that logic, thinking about replacing Koeman with Unsworth is suicidal. I bet he will be gone within the 6 months of his tenure. All his reputation with U23 will be tarnished cos the man has 0 experience managing a 40k stadium, players on huge salaries and transfer fees and keeping up with the PL pressure. Point is, impulsive sackings are not always for the BETTER. If you want to sack a manager, give him reasonable amount of time and then when you do, be 100% sure about the man you want to bring in, back him 100% for a certain period of time (not 4 months or 6 months) to see if he can deliver. And while doing that, the board and DoF which generally hide behind the curtains in tough times need to take some responsibility.