I do have to question how you are "easily" getting prime attack ability to +5. It starts at no better than +3. You would need TWO stat increases to get it to 20. Which means you didn't spend those on feats. So it should be taking a long time to get to that point.
A typical monster may have an AC of say 15, so with the Prof Bonus the player would need to roll an 11 to hit. Great, he should be hitting half the time. Once you roll in other bonuses however things, in my opinion, go south. It's very easy for a player to max out a characters prime attacking ability to +5, add in anticipated magical bonuses of +2 by 9th-14th level and the character suddenly has a total bonus of +11 so he now only needs to roll a 4 to hit the typical AC of 15.
This isn't what I was really expecting from 5th edition so I've started think of ways to fix it.
What do others think, should monsters become this easy to hit by mid levels? A creature with an AC of 12 can only be missed on a natural 1 so is hit 95% of the time. This makes taking feats that give -5/+10 damage a no brainer as you'll still hit most of the time. I was hoping for more!
There is a faulty assumption here: Most PCs should have a +5 attribute by this point (although not all), and the +4 proficiency, but assuming a +2 magic item is not a safe assumption if you use recommended item distribution.
5E doesn't have bounded accuracy, it was just something mentioned as a nice to have during beta but it never made it to release, it is as simple as that.
If they use up all of their resources (and a share of the cleric's - bless is also concentration), then they can nova their chance to hit. However, a +2 AB requires a 4th level casting of magic weapon (available once at 13th level for paladins) and is concentration for an hour. Accordingly, assuming it for all combats would be hasty - and sacred weapon is a channel divinity that lasts one minute and grants Chr bonus, which may or may not be as high as +4 at the low teen levels... Paladins have a lot of need for feats, strength, and constitution. Further, there are a lot of other paladin spells that require concentration that are combating for that benefit. If the paladin Novas their bonuses they might get up the +16/+17 range... but I think the rest of their combats would be back down to the +9 or so range. Further, we're not just talking paladins here - fighters, rangers, rogues, etc... do not necessarily have access to bonuses to hit as easily.It is an extremely safe assumption thanks to the magic weapon spell. Your paladin will have a +2 weapon if they want it, and if they're smart enough to go Devotion then for that BBEG they're also guaranteed another +4 or +5 to hit from sacred weapon. +4 str +4 chr +4 prof +2 magic weapon +2.5 cleric's bless = +16.5
I noted that as well...And lets not forget he was using a melee weapon, were he talking about a bow then archery master hands you +2 to hit on top of the magic weapon for free!
It is an extremely safe assumption thanks to the magic weapon spell. Your paladin will have a +2 weapon if they want it, and if they're smart enough to go Devotion then for that BBEG they're also guaranteed another +4 or +5 to hit from sacred weapon. +4 str +4 chr +4 prof +2 magic weapon +2.5 cleric's bless = +16.5
And lets not forget he was using a melee weapon, were he talking about a bow then archery master hands you +2 to hit on top of the magic weapon for free! 5E doesn't have bounded accuracy, it was just something mentioned as a nice to have during beta but it never made it to release, it is as simple as that.