There seem to be two arguments as to why spellcasters aren't necessary,
The first assumes that adventures are designed by the GM, with the basic trajectory of action decided in advance, so that the GM will make sure, via the design plus downstream adjudication, that any party of whatever composition can travel along the pre-planned trajectory.
The second assumes that players will establish wants and needs for their characters commensurate to their capabilities - so if the PCs can't teleport, or send instantaneous telepathic messages, or travel to other planes, etc, then the players will reconcile themselves to doing other things.
The second seems largely tautological. The first is a very specific assumption about the approach to play which is certainly not true for me.
When I GM FRPGs the players want to do things that exemplify the "F" - fantasy! And the main way they do this is via magic. There are all sorts of pathways to that, both in terms of the fiction and the mechanics, which vary from system to system.