77.5% is definitely a "dominant market position". That is the term that matters, people just like to use the word monopoly instead (note: if lots of people use a word "incorrectly" the meaning of the word changes).
That differs between the US and Europe. The legal issue is abuse of dominant market position (just having the position is, of course, legal). The EU cares about any "detriment" for customers, competitors, employees,... . That used to be the case in the US as well, but over the last decades they switched to caring only about a specific niche and generally doing very, very little.
End result: Lots of big, successful companies that do shady things and only the EU intervenes and then is the bad guy.
Look at any of the threads about fines for Facebook or the like and you will find many replies to the effect of "The EU is just jealous and should not fine successful American companies".
