That's a far fetched comparison. Contrary to Latin there was no "downfall" and there are still English speaking national institutions codifying a standard for school education. (tho I have to admit that phenomenons like Trump and BoJo do their best for a collapse)

> only several different incompatible dialects.

Those existed already in Roman times, almost all citizens were illiterate and spoke only a mix of vulgar Latin (Roman street slang) and old local languages. The lingua franca of the eastern empire was Greek. Later invaders added their languages to the mix (e.g. Germanic words like "bleu", "nord" or "boulevard" to French).

Furthermore are those "school standards" only relatively recent in wide-spread, during the first world war many military units had to be formed from the same region, for the soldiers to be able to communicate.

What's happening now is not a decline of the standard, but the visibility of many sociolects and dialects thanks to media and internet.

50-100 years almost only standards had a chance for mass distribution. Today you only need a smartphone to spread your lingo.