This "everyone guard the caster" is not a role in the game. Tanks guard
everyone in the rear while forming the "front line", including ranged support, whether spellcasters or martials. Healers heal everyone. Utility casters do whatever is most helpful. Skill monkeys do things no one else does. Scouts scout and infiltrate. And on and on...
Even in their indiviudal roles, everyone contributes to combat in a meaningful way and is important.
We just had a newbie join our group last night. He's played BG3, but no other D&D at all. In our group we have:
- Fighter (tank and reliable damage)
- Paladin (NOVA damage, less reliable otherwise, not as tanky)
- Rogue (scout and ranged damage)
- Sorcerer/ Cleric (healer and tank support-great AC and HP)
We're 6th level, and he said he wanted to play a monk. Given what we have I explained he won't have the HP or damage potential to tank or nova, he could be a good scout as well, etc. However, his main role will be in mobility, support, and lockdown opponents.
Another option I presented if he feels okay with a spellcaster (as a newbie) is the AoE, buffer, or artillery caster.
Either role will be valuable and an important to the group. Either as a non-caster or as a caster.
Earlier editions had very sufficient checks and balances for the "power" of casters compared to non-casters. Not that it's needed, really, since PCs don't often (and shouldn't IMO) go toe-to-toe against eachother. Otherwise, its a team activity and players should celebrate each other's moments instead of wondering "when's
my turn!?"