While "concentration" is part of the 5e rules, i still wonder who thought that up. Or at least decided which spells require concentration and which not.
Since this is my first contact with 5e rules i was...surprised...to say it mildly. There are some spells/rituals where i can understand and accept such a mechanic but to think that a cleric can bless 3+ people OR give one person a Shield of faith is...BS

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Spells like the "hold" line get ridicoulous this way. The victim gets a save each turn AND in addition his friends can just pummel the caster till he fails his CONSTITUTION save. Most casters surely have tons of that...
(Just fyi some enemies get their save at the beginning of the turn and can even act when they succeed, i forgot which fight it was but it seriously annoyed me. i think it was that "spike" guy)
So if some rules from 5e are ignored or changed please let this be one of them. Or keep it for spells that need actual effort AFTER the casting. Divination for example.
It really grinds my gears to see that you can conjure a blob of magical grease and then look elsewhere but to conjure a for of magical darkness you need to concetrate until you get slapped or the time runs out? Eh?
I’m not exactly sure I understand your argument. Are you arguing that magic doesn’t follow a logical system. Your first example: bless is a action vs shield of faith is a bonus action. Mechanically, you’re choosing between one person or many. In TableTop, you would also have to consider spell components. Are you arguing that magic is arbitrary? In that case, I think you have to accept a level of abstraction due to the innate nature that is magic.
When you cast the spell hold, you take one entire character out of combat. Action economy is important and not having one character can destroy a team, additionally why wouldn’t thinking creatures attack the target that’s causing them to lose? Especially if it’s easy to hit (By this I mean I never had my Magee attacked because I use my movement to get them out of harm’s way when attacking.)
I think that if it bothers you that grease isn’t a concentration spell then every spell would be a concentration spell by default. The reason grease isn’t a concentration spell is because you take your spell components and turn that into the grease, you are transmuting their properties. You are actually concentrating on suppressing in the other spell.
Overall, it seems like you just don’t like how 5e does magic. That’s completely fine! You have to abstract magic to make it fit into a fighting and this is the method they choose to allow some flexibility while not letting wizard’s essentially build save or suck characters where if you fail a save, the other character wins which was a big problem in pathfinder and 3.5. With that being said, how would you balance the game once casters can use spells like black tentacles and then start laying on other spells like Otto’s irresistible dance?