I would probably be more ok if you could have, at the bare minimum, one offensive concentration and one defensive concentration active at the same time. This would let me help out my allies while still allowing me to play the type of Mage I enjoy. Maybe a custom feat, although it just seems like another feat tax.
Its interesting how everyone always talks about "the earlier editions" in this argument, when they are really just talking about specifically 3E in order to make their point. Yes, casters were unbalanced in 3E, this was due to a plethora of reasons, not the least of which was removing the drawbacks of magic. They also gave the previously warrior exclusive multiple attacks to everyone whilst removing his advantages (superior saving throws, exceptional strength, exceptional con, better magic item access, etc). Not to mention there was no such thing as a concentration save in AD&D, if you got hit in the process of spellcasting you automatically lost the spell. All of those things combined (and more) is the reason we got the unbalanced nature of things in 3E.
And because the question was asked, I very much love the team concept of D&D, i'm always the first one to congratulate someone on a good idea, give encouragement, and celebrate other peoples natural 20. I'm not looking to overshadow anyone or relegate my party members to glorified henchmen, then again this has never been the case of our group anyway. Although I love the mental exercise I get from coming up with clever uses or combinations of spells to tackle a problem we face, it is part of my enjoyment of the game. Sometimes being part of a team also means letting your fellow teammates take center stage, whether this is the Rogue using stealth, deception, and thievery to successfully break into the sultans treasure vault, the Bard using a combination of bardic performance, diplomacy, and roleplaying to bypass an encounter completely, the Barbarian going through swaths of enemies in melee, or the Mage doing his thing with his spells, it's ok to sit back and enjoy the show
The funny thing is, I love almost every part of 5E, from the focus on roleplaying to capturing that intangible "feel" of D&D, my only two real nitpicks are the aforementioned magic system, and going from 1 hit point to 100+ with 8 hours of sleep, other than those two things they nailed it.