By this logic wouldn’t our fans have been calling for Klopp to be sacked after his first season/second season?
To be fair, Liverpool had accumulated 40 points with 45 goals scored after 18 games in Klopp's second season at the club. And the first ½ season ended on a somewhat optimistic note with them beating us to ultimately the reach the Europa League final with only 1 loss since the start of Gameweek 30 in the league. Solskjær's United isn't on a comparable plane because we are far behind with 25 points and 26 goals scored after 18 games in his second season at the club. And last season we went into a tailspin after the win vs. Paris Saint-Germain...losing 5 matches since the start of Gameweek 30 in the league (which left a distinctly bitter taste in the mouth).
Furthermore, you could see the rapid progression in their structures/style as they absorbed Klopp's instructions after just 2-3 months — this aspect is kinda abstract and subjective, but probably even more important than raw point/goal totals because once the basics are laid and optimized and a consistent system is fostered, you see incremental improvement with automatism and the acquisition of better (or more suitable) personnel. After a year under Ole, United play with a vague and disjointed style, and that lack of cohesion and consistency should give even his most ardent supporters a bit of pause. Even at a club like Borussia Mönchengladbach, which appointed a new manager in the summer, you can see the quick transition to Rose/Marić's style — while they sit 2nd on the Bundesliga table after 17 games.
Additionally, you have to consider Klopp's background when he was given the Liverpool job — while Dortmund were somewhat dysfunctional in his last season, he had already won 2 league titles in a Top 3 league, reached a European Cup final while beating Real Madrid, and left an imprint on the era...even experienced managers like Heynckes were using some of his principles with their teams (ultimately leading to the Treble at Bayern). While he wasn't a pioneer he was definitely someone who advanced the tactics and preparation for his generation and Liverpool's board-members and fans could trust him because of his history in some of the highest echelons of football. Solskjær's CV before the appointment at United was objectively much weaker.
There is some merit to Ole — he isn't bad in an absolute sense — and a lot of us want to be optimistic in a romantic leap-of-faith way because he clearly loves the club, but his position will become untenable if we don't improve in quick order. And it wouldn't hurt to at least sound out potential replacements, instead of being cocksure and then scrambling if things go tits up — which is unfortunately our modus operandi under current administration going back to Fergie's retirement where we didn't even have the semblance of a cogent plan. Mostly because of 2 reasons:
1. Top 4 is still within our reach if we go on a run or undergo a new manager bounce/rejuvenation. If we don't improve and someone who can become a medium term manager is available (or extractable from his current club), we should be proactive/decisive and move quickly while the season is salvageable. Missing out on Champions League qualification once or twice is pardonable, but the 4th time in 7th seasons for a club that considers itself on the same level as the contemporary European elite would send out poor signals of our sporting ambitions/growth and also lead to significant financial penalty.
2. If a new manager trains us for some games this season and has first hand experience with the players (including their personality and characteristics), he will enter the summer with a decent basis and actively consolidate in the market and training camps. If we simply limp towards the end of the season and then part ways with Ole because of some clause (like Champions League non-qualification), the new manager will enter the summer with very little first hand knowledge, and will take a few months to figure things out (which will set us back next season, too). We should be very, very careful and methodical when the upcoming summer window will be harder, or more competitive, than usual because of the European Championship.