Arsenal's reserve team is indeed not strong enough and it is true that you said that if the reserves were strong, it seems that the potential to become champions is more open for Arsenal. but for some people who are bored with seeing Arsenal who have not become champions will definitely be annoyed. but of course in my opinion it is not easy for Arteta to be able to compete in the EPL because of course the other teams also have very good squads. so for me it seems too early to say that Mikel Arteta should be fired and I think that Arsenal who are currently leading in second place in the standings is also a good thing and of course it is possible that from now on Mikel Arteta will train his men well and of course will compete closely with Liverpool. but it is true what you said if for example Arsenal dares to pay good players and dares to spend a lot of money then Arsenal's great potential to become champions will be realized.
considering Mikel Arteta since joining Arsenal Arsenal has made progress and of course it is a positive impact of Arteta's presence but in my opinion Arteta could be given a chance this season if for example this season Arsenal does not become champions of course Arsenal can fire Mikel Arteta.
Arsenal failing to win the league is as common as it gets, been true for three decades, whenever they looked good they ended up playing against some team that is better and end up failing. The most common issue they have is that they end up losing near the end, their momentum doesn't carry over to last third of the season and because of that they end up losing and that's the most important part.
As long as Arsenal could just play the whole season at the same level, I am pretty sure they could have won at least one title, but they always start great and then get worse gradually. If they had good subs and a big depth then they would have been better, it's clear that they faced so many injuries, and because of that I believe we should not judge them, injuries do make a team weaker, but if they had a proper depth then they wouldn't have this, they could have done better without a doubt. Unfortunately they are not looking that great at the moment.