That's grammatically correct.
That depends on how you analyze it. From a grammatical standpoint,
Starbucks and
McDonald's are not plurals as words.
I regret that my use of the old-fashioned word "aplenty" in an example seems to have caused people to disregard all the main points in my first post above.
The best defense of
Starbucks and
McDonald's being plurals is that they
could be followed by a plural noun which they
would modify IF it were there:
many Starbucks cafes
many McDonald's restaurants
Notice that even in those constructions
Starbucks and
McDonald's are not plurals; they are attributive nouns modifying plural nouns.
Countless company names get pluralized for plural reference:
Toyotas, Hondas, Hiltons, Motel Sixes, Safeways, Burger Kings, Jack in the Boxes, BevMos, etc.
But when the company name ends in
-s, there is a resistance to doing the logical grammatical thing to pluralize the name.
Different native speakers will have different reactions to, say,
a number of Toyota Priuses/Four Seasonses/Lowe'ses.
Lawlessness reigns here, grammatically. The "rule" for native speakers seems to be: "If it sounds good, do it; if it doesn't, don't."