I've never seen an oldschool Power Rangers fan bash the newer gens of Power Rangers.
You've clearly never been on a Power Rangers fansite. There are certain Seasons that are routinely bashed.
Linkara also did this on History of Power Rangers with Turbo and Super Megaforce (actually, most of the ones Nickelodeon has done have been ridiculed by him in some way, and for good reason).
I've seen Star Wars fans complaining about episodes I, II and III (even if they were born much later than IV, V and VII debuts), but always rationally.
You should have seen Spoony's Knights of the Dinner Table review of Episode I. He ripped it apart, even back when it first came out (trivia note: when Phantom Menace first came out,
people actually LOVED it; I think the reason is because we were so starved for a new Star Wars movie that we didn't care that it was horribly flawed and that it tried to rewrite the entire cosmology of the universe, and nowadays it's pretty much universally panned as being the worst Star Wars movie ever made).
And let's not forget how people talk about how much Anakin whines in Episode II (seriously, it's on the level of my criticisms of Tidus from FFX in the whiny department).
The Pokémon Genwunners are different.
Like the genuine genwunner I know, in this arguing that happened 2 weeks ago. Both me and him were in a group at the college discussing something, when a girl in the group showed a meme related to that discussion, but pictured with Pokémon. Since the color scheme was important, most pokémon used were from gen 3 or higher. He could have said nothing. He could have said "I don't know those pokémon". He actually started with "If those pokémon are from another gen than 1 and 2, I don't know them". But he had to start the bashing, how every subsequent gen was "getting worse", the classic "a f***ing garbage can" and all. He tried to appear rational saying "I know there are bad pokémon from gen 1" and even "I've seen all the new pokémon, I even liked some", but failing to recognize the nostalgia bias. Instead, he started to call the Pokémon fandom the worst fandom ever (lol).
Every time someone talks about newer pokémon is the same thing, he always bashes the new generations.
Of course, I know much more people acting as if there were only 1 (sometimes 2) generations, but I don't call them genwunners. Of course I get sad when my Japanese classmate says she's a "ghost trainer" for Pokémon GO while knowing only 3 ghost-type pokémon, but what can I do if she's not interested? While she doesn't insult the new generations (like the previous guy ALWAYS does when he gets the chance), she doesn't fit the label.
The person you mention though is just one person. I don't think an anecdote justifies putting a label on entire section of the fanbase. That would be like saying that all Pokémon fans are creepy, because there's inappropriate pictures of Pokémon on the Internet. Obviously, this isn't the case either.
Also, I would hope your instructor put a stop to that. Otherwise, I would seriously question their competence in controlling the class.
On the last point here, that's the nature of Pop Culture Osmosis. The further along you get in a franchise, the fewer people there will be that are familiar with the newer parts of it (for reasons I mentioned before; they lose interest or they grow out of it). Power Rangers is one example, but the same can be said about Star Wars and how most people know who Darth Vader is, but considerably fewer know who Asajj Ventress is. And similarly, most people know who Captain Kirk is, but fewer people are going to know who Captain Archer is. Both of these examples come from franchises that have lasted a very long time. It isn't that those people aren't 'true fans' or that they are 'haters' when it comes to the newer things, they're just not as well versed anymore.
Also, keep in mind that if it weren't for a lot of these so-called 'genwunners', Pokémon wouldn't be as big as it is today. About half the people playing Pokémon right now (more likely over half), were not alive when the original first gen came out (or the second). And if it weren't for the older fans of Pokémon latching onto it in the first place, we wouldn't be sitting here talking about it now and this site wouldn't exist.