Unfortunately, if you don't have a frontline, then your squishies ALL become your frontline. Which I think we can all agree is bad. You can argue that a team of all wizards could account for this by all packing shield and other defensive/control spells, but then you are really cutting into their DPS and eating through their spell slots.
I don't know if you are referring to TT as I only play PC games but to me 'frontline' and 'squishies' are concepts rather than realities. Classes in 5e have so much overlap that there are generally options for filling a role. Add to that the fact that you don't always get to enter the combat with you 'frontline' at the front In BG3 Shadowheart can do the locks/traps (at least in EA), in Solasta I use a Ranger for the role. Bards were generally viewed as a viable alternative to Rogues but I don't know if that has changed in 5e. To be honest I view Rogues as redundant now. Their only selling point is sneak attack but that requires too much micro-managing for me and from what I've seen in LPs on YT they spend more time trying to manoeuvring than actually hitting anything. My current Solasta party has a paladin, cleric and sorcerer who can all chuck out fireball. The sorcerer never bothers with Mage Armour of Stoneskin as her AC is pretty tasty with bracers and an amulet or ring. Ranger can do 3 attacks per round either with bow or DW. Her Spike Growth has won us more fights than Sneak Attack ever will. Her HP is the same as the pally's . Cleric can do 2 attacks per round from level 8, uses a longsword (elf) and wears medium armour and shield. Shed load of useful spells including Guardian Spirits (or is it Spirit Guardians). Pally, cleric and ranger all have healing spells. The only real 'squishy' possibility is the wiz or sorcerer especially early game but Misty Step can get you out of the fray. Clerics can wear medium or heavy armour and rangers get medium armour prof.
My other, original, Solasta party is similar except for wizard instead of sorcerer and a different flavour of cleric (heavy armour but only one attack per round. Biggest downside I discovered with the new party is that sorcerers can't use the Identify spell nor can anyone else in the party.
How this party composition will fare in BG3 or if it will be possible remains to be seen. Ranger and sorcerer seems problematic at this point (please don't mention Minsc).
I haven't played Solasta, so I'm not sure which subclasses are available, but sure, at this point in 5e pretty much every class has a subclass that can take/dodge a few hits. Other than Swords Bard, BG3 doesn't have the subclasses that make it easy though (hexblade/bladesinger/etc.). I do doubt that you could leave your sorcerer up front adjacent to a group of hard hitting bugbears with goblin archer support and last long though, even if your paladin is in back throwing fireballs at things (which sounds suspiciously like multiclassing). Also, building a team based on access to a ton of powerful magic items isn't really how 5e is built.
The crux of my point isn't that you can't build a 4 person party of all evocation wizards and storm sorcerers, it's that that is something that isn't easy and takes a lot of planning and thought. You may be the greatest DnD player out there and could pull it off with ease, but min-maxing isn't something everyone is good at.
