There is very little precedence for a situation like this to make any kind of call. There have been very few clubs of our size taken over in the last 20 years or so. City and PSG aren't good comparisons as their starting point was far away from ours. The only comparable club really is Chelsea after Roman was made to sell, but then the people that were placed in that job (At least Grovanovska or whatever her name was) were personally affiliated with him. Arnold doesn't seem to be a personal Glazer man like Woody was.
Either way, we have a very small sample size to go on so its quit pointless making assumptions about what will and won't happen, especially with such confidence.
And if he happens to get replaced and we end up getting someone better, great. By and large him and Murtough's "good work" has largely been based around getting the most exciting managerial prospect in the world in at the club. Had we gotten Poch in and were floundering in sixth place, then nothing they were doing would have been receiving any praise.
It's a bit like how FSG and the Liverpool hierarchy suddenly all became great at managing a club because they got Klopp in and he got things going in the right direction.