That's one problem. The other problem can be described like this: assume you have two tiers of spells - tier 1 with the most useful spells and tier 2 with less useful spells. Without concentration, you'd naturally cast the most useful spells first and then consider whether you need to cast the less useful spells on top. With concentration, you suddenly cannot cast tier 2 spells on top of tier 1 spells and the result is such that tier 2 spells will never get cast or will be cast in extremely limited niche scenarios.
Because the point, BY DESIGN, is precisely that you are supposed to pick the ONE concentration spell that you consider situationally more useful and then everything else you cast (with the same character) should be your non-concentration ones.
If you are a cleric, for instance, you'll probably make your initial choice between bless, spirit guardian or Hold person (depending on what spell slots are unspent at that point and what you are facing) and then you are left with guiding bolt, spiritual weapon and whatever else your specific vocation offers.
You are talking as if this was an unintended consequence, when the goal is SPECIFICALLY to not make some synergies too trivially easy to pull off and pile on top of each other.
Sure, you can use Web and Cloudkill together at the price of having two different casters occupying their concentration slot for it, otherwise you'll be content to pick the one you prefer and then use every other spell that doesn't ask for concentration as a requirement.
We could argue that some spells asking for it are highly questionable. (i.e. Hunter mark, which is so core to a hunter? True strike, that already borders on useless even without it? Etc) but as a general design principle it's absolutely solid.
