Unfortunately, by simply auto-ending possession, it side-steps completely the iconic battle of wills between the exorcist and the possessing entity. While forcing a save or requiring a touch attack doesn't do very much for that idea, it at least conveys the idea that the exorcising effort is opposed by the possessing entity and can fail the first attempt.
Sure but ok, how many spell slots can the player actually come up with here?
D&D magic is magic bullets fired from a gun. And often, there are situations that only have magical solutions- I'm not saying that's great design, but that's just how the game
is. You ask a character to end possession, there's precious few ways you can do that. If you swing and miss, then what?
It's not like this is some static ability you can employ turn after turn in this back and forth battle like you'd see in other media. I can understand not wanting to instantly solve problems, but again, that's how D&D magic works.
You'd have to redesign the magic system from the ground up to do it any other way. Make "dispel possession" some kind of Ritual that is performed once you have the possessed victim tied up or something.
I once had this situation where another player was petrified in combat. I tried to use Greater Restoration to fix the problem, because that's what the spell does. DM tells me "oh, it doesn't work, you can't de-petrify someone until you kill the monster that did it".
So great, now that player has to roleplay a statue and hope we kill the bad guys so we can get him back into the game- oh wait, we can't, because I just wasted the Greater Restoration I was going to use, so even when we win, he gets to wait until I can rest and get the material component. All because it wouldn't be "fun" to let something actually work?
I mean how fun is it for the player, who just happens to have the right tool for the job when the situation comes up, to be told "nah, bro, can't let you do that, it wouldn't be exciting".