He's right, though, and this is coming from someone reading the greeks/romans to a borderline autistic level. Obviously every classic, especially from antiquity, ESPECIALLY classical Greece, will inform much of your subsequent reading. Personally I think that makes most of it worth reading. But if someone sees fit to read Homer, some Plato, maybe Herodotus and a few plays, they'll probably be fine. They'll miss some hints and references, and I 100% agree that it will at least to some degree lessen their later reading experiences, but if someone doesn't love the Greeks there's no need to force them to read the classics in full.
>now no other classical work will seem as long in comparison
m8 there are numerous primary source histories from antiquity easily 2-3x as long and far less approachable
buckle up motherfucker
Just a quick word of advice: don't bother trying to plan so far ahead. It doesn't work, because it can't work, because you haven't yet read the things you're trying to plan around, so you don't know what time periods they'll cover and which ones you'll need to fill in elsewhere. It's easy to whip up a list of 6-10 writers in reference to the Greek or Roman chart, but if you actually diligently read those few you're starting off with, you'll stumble onto references to other interesting, valuable writers as well, whom you may very likely wish to check out.
As a few examples, you might finish Thucydides and realize his books end before telling how the Peloponnesian war ends. Oops. If you want to wrap it up with a primary source, you have to read Xenophon's Hellenika.
Or you might start reading about Rome and realize that the Aeneid is way more important to have under your belt than the Argonautica.
Or you might look at the ToC of Gibbon and realize most of the history takes place after what you think of when you think "ancient Rome." Might want to read some Dio, maybe some Procopius, maybe some Byzantine history.
Or you might read all of Livy (and you should, he's great). Turns out his extant works tell you nothing about Pyrrhus (better read Plutarch) or the first Punic war (better read Polybius), or the perceived moral degradation of Rome after the fall of Carthage (better read Sallust), or the birth of the empire (better read Caesar), but oh fuck even Caesar is just a latecomer in a long series of 1st C BC civil wars (better read Appian) and the empire par excellence doesn't even come about until Augustus (relevant books in Appian lost; obviously outside of the scope of Caesar's books; better read Dio Cassius).
Obviously you don't need to actually read all of these guys. My point is just to take it one step at a time and do not even try to set a 10 book plan or whatever. It won't work. And it shouldn't. If you discover no new, exciting, possibly minor authors along the path of reading Thucydides and Livy, that will be the saddest possible conclusion of the very noble plan that you have so far.