Well there is a difference between two sorcerers and two wizards vs a 'all spellcaster' party - druid, cleric, wizard and sorcerer are all primary casters. Add to the group bards and warlocks and you can easily setup a group of 4 primarily casters in 5e that will work. At the same time you could go without primary casters and it still is viable in 5e. Since you don't need designated healers or mages I would feal very confortable going with a ranger / paladin / druid / bard without min-maxing and playing with multiclasses for example with no worries because 5e isn't based as heavily on buffing and de-buffing as older editions. The system also doesn't truely support traditional MMRPG roles with tanks/healers/dps - you can't force enemies to attack tanks, healers don't heal efficiently, but there are control and dps builds to be fair.
Don't get me wrong you can create parties that will have a harder time, but its still far more forgiving than other systems.
First off, every bard I've ever played is personally offended by you leaving them off the primary caster list :p
Second, I agreed that 5e is more forgiving than games like WoW
Regarding your specific examples, I wouldn't ever label a Cleric as squishy, they are the only full caster that easily gets access to heavy armor and all martial weapons. Druids basically have access to massive amounts of temp hp (for now) with their wild shapes, so I wouldn't consider them particularly squishy either. I've seen both of those classes act as frontliners. In fact, all of the moon druids I've played with pretty much only considered themselves frontliners, including a multiclass 'bearbarian'.
I suppose to me squishy = d6 hit die and limited access to defensive measures that don't eat into their dps capabilities, though even some warlock and bard subclasses border on squishy. Whereas a tanky frontliner isn't paying a huge dps cost to have rock solid defenses e.g. bear totem barbarians or shield bearing Paladins (especially since PAM works with 1h spears now)
