Is there a Class Designer's guide somewhere that spells that out? Is so, link it. If not, don't pretend that there's anything so bizarre as a requirement for single-player or single-class party viability in an essentially cooperative game that runs on spotlight balance.
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I know 5e has a rep for being 'too easy,' but if you do follow those guidelines, you'll end up with 6-8 medium/hard encounters (by including some encounters where the party is outnumbered, resulting in hard encounters with less-than-hard exp value), and single class parties will likely choke on that, especially indifferently optimized ones at 1st level. Some of the more versatile classes, though, you could optimize a party of them to cover everything it needs, and, if you heavily optimize a single-class party it could probably blow it's way through at least a full day of standard encounters, even if they're all hard.
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Again, that is a baseless assertion. You'll need to find it in print, or get a designer to swear to it, before I even start to take you seriously.
I mean, Mearls /is/ on record with goals for 5e's inclusiveness and integration of all past editions to the point different players could enjoy the 'styles' of different editions, at the same table. No one takes that seriously.