Flexibility is key yes, but spellcasters have to make choices, typically at the start of the day, on what to focus on and what not to.
Agree. When discussing theoreticals, I too have noted that any theoretical caster has everything he needs. They really won't they have to make choices. So at somepoint they could make the wrong choice. The thing is they have choices at all. So you're right, overall.
However...
The last paragrapch of your post went into somestrange land about banning armor spikes. Its was very LOL,WuT?. Cause that means someone in that group doesn't know what they're talking about mechanically and are banning things that are absolutely Unbroken. I started to do a line by line but its not even that crucial, banning archery focused builds ends with only casters being useful in ranged combat beyond 60ft on an open plain. Which is sad.
So long as you avoid the really cheesiest builds, a balanced party still works better than all spellcasters
This is totally contingent on how you define "cheesiest" it sounds like your definition includes any caster whose dedicated to doing anything but evocation.
An all caster party works better than a balanced party if the the All caster party is balanced. I could show you how they work better in every way if you like. Though if you're arguing that anycaster that is gishing, is cheesy, you've insulted gotten rid of many favored archtypes for many people and some our favorite chars of all time.
Frontliners is a very wierd term but you have to understand that the job is pretty easily replaced via Caster. Further well it should be, the option at least, cause there are monsters in D&D that NOBODY should engage in melee.