I don't think multiple concentration spells necessarily breaks action economy. If you can cast both Longstrider and Jump on someone (without concentrating at all), why can't you cast Bless and Shield of Faith? I thought it worked fine in previous editions when spells just had durations. Hold Person doesn't need to be a concentration spell, the target already gets a save every round, it might fail to even land, or it might break on their first turn, you're unlikely to get so lucky as to have it last more than a couple of rounds. It seems really arbitrary to me which spells require concentration and which don't. Spiritual Weapon actually does break action economy by basically giving the Cleric an extra attack every round, and it doesn't even require concentration! It just lasts a minute. It completely destroys whole character concepts. Want to play as a buffer? Too bad, cast your one buff and then shut up and cast Guiding Bolt. Want to play as a crowd control character? Too bad, cast your one control spell and then shut up and cast Scorching Ray. It just limits casters too much. I'm fine with it being harder to maintain concentration on multiple spells, that seems reasonable. But just not being able to use most of your spells because you're already concentrating on one feels bad.
Its easier to illustrate this with video games than using a tabletop example and fortunately, since this is a video game and not tabletop, it also makes my example even more justified :P Lets assume for a moment you could concentrate on multiple spells, how would you, as a player, be able to take advantage of it? Simple! Cast all the buffs you like outside of combat on the party, then initiate combat. Once everyone is in combat, throw on something like invisibility on the mage and put them more or less outside of the action so they are unlikely to take incidental damage and now you are fighting a battle with a significant advantage from the outset. The end result is combat becomes trivially easy for anyone who does this. Well, what is the problem here you might ask. The problem is, that if you want to have a difficulty within the game that challenges players who do things like this, that difficulty option immediately becomes unbeatable for most players who do not.
This is incidentally why Sawyer made buffs expire automatically outside of combat in Pillars of Eternity - he was trying to solve this problem. Now while I do not agree with his solution, I do acknowledge that the problem exists.
Now, how does concentration fit into all of this? Well, concentration as a mechanic has the following advantages.
1. The mechanic has verisimilitude. You can believe that a wizard who is trying to create an effect which persists, would need to concentrate on it in order to keep it persisting.
2. The mechanic solves a balance problem. Buff stacking is a major issue in video games which creates a gulf between players and by adding the mechanic, this gulf is made much smaller.
But it also destroys an entire playstyle. The buff wizard (which is incidentally, a playstyle I personally love to play), basically does not exist with the way concentration is implemented in 5e. I don't actually consider that to be a fault of concentration though, you could modify the rules of concentration to achieve some degree of balance while still allowing buff stacking, it just makes the rules a lot more complicated in the process. Its fairly obvious though whatever you implement, it needs to have a significant downside. Aside from things I have mentioned already like making you roll with disadvantage if you split your concentration between spells, some other possible drawbacks could be.
1. A Metamagic Feat called Effortless Concentration. Allows you to cast a concentration spell without concentration, but casting it raises the level of the spell by 3 spell levels (so for example, you would be casting a level 1 buff in a level 4 spell slot with the level 1 effect, but it won't take concentration).
2. A feat called split concentration, which allows you to concentrate on 1 additional spell, at a cost of having to ban an entire spell school.
I could propose other ideas, since I like an excuse to get creative, but seeing as I very much doubt they will be implemented, there isn't much point and they can stay in fantasy land :P
