I think that even Alabama fans understand that nobody was going to win at the same level. On average Saban won a national title a little more often than once every 3 years. Until 2020, he actually won a national title every other year, on average. He won the SEC at a clip approaching 2 out of every 3 years (9 out of 16 seasons he was at Alabama). Even the most delusional, entitled Alabama fans know nobody else is going to come in and keep that pace up.
I think what they expect out of DeBoer (or anybody who happened to replace Saban) is being one of the top 2 or 3 SEC teams in any given year, winning the SEC once every 2-3 years, making the CFP basically every year, every 2-3 years making a deep playoff run, and eventually delivering on one of those deep runs by winning it all. High expectations, yes, but they should have high expectations.
What they are seeing now is so far removed from being able to meet those expectations; that is why they are upset. They look like what I would describe as a "typical good team." There's absolutely nothing special about them.