Pretty sure the definition of readable covers what you are saying. Things don't have to be necessarily a word or legible to be readable.

For instance, if I put two railroad spikes together in a off set plus and ask a Christian, "Is this readable as a Christian symbol?" That would be acceptable.

they’re doing a disservice to themselves by simplifying their logos to plain text.

It also dilutes their branding when translated into different languages/writing scripts.

That McDonald's 'M' is instantly recognizable in every language, even languages that don't use Latin-derived letters.

But if your logo is plain text, how do you translate it into Cyrillic or Kanji? You either have to develop a completely new logo for those regions (hurting your worldwide recognizability) or you just roll with the English logo and have to deal with most of the locals seeing it as completely unintelligible.

Logos don’t need to be readable.

Completely depends on the circumstance. Do logos of global icons like McDonald's, Apple or Volkswagen's need to be readable? No. Of course not because they're everywhere. Does your local electrician's logo need to be readable because it's literally just their family name? Yes. Yes, it absolutely needs to be readable.