Starting to mess around with what are concentration spells is a quick way to unbalance DnD. You are correct that a 9h level cleric will not be wanting to concentrate on a bless spell, here are the reasons why that is not an argument to circumvent that spell being a concentration spell:
1. The cleric will be concentrating on a more powerfull spell, the class is balanced around getting more powerfull spells, not around stacking a pile of concentration spells on top of eachother.
2. By lv9 the party will have a significantly improved stats and magic weapons helping them with their chance to hit stuff so bless will have less impact.
3. Clerics can swap out what spells they have on a whim so it "costs" them noting to pick a spell that is good at low levels and then replace that whenthere are more usefull things to do. This is a general thing with many DnD spells, they are simply good at certain levles but less usefull at higher levels.
