You're just familiar only with the best titles from the specific era.
If you think that modern anime is garbage, but the old-school titles are all bangers, you are wrong, and that's completely understandable, because that's how the mechanisms of nostalgia and history work.
The human psyche has a lot of defense mechanisms and subconsciously tries to guard you from the bad stuff you encountered in your life. Sometimes it backfires, but most of the time it works pretty well: you forget things that you didn't like. That's why you think that back in your childhood the food was better, the grass was greener, and the anime was brighter and more interesting.
Also, don't forget that people have been actively using the internet and sharing information with each other only for 20-25 years. Right now we are bombarded with new information every second, we can't hide from the news, can't escape the hype, and some of us, the most devoted fans, are checking out most of the season's titles. And the statistics are unrelenting: even if your taste is quite broad, you will encounter things that don't fit your palate, just because you're going through a huge mass of data. And the more titles you go through, the higher is the probability to end up seeing something you wouldn't like.
It's doubtful that a person who boasts about anime peaking in the ‘80s or ‘90s or any other decade actually watched every single title of that praised decade. Most of the time they just studied the classics, the most popular movies and series, quite religiously, but never thought about all the shows that ended up not being popular and were forgotten — and thus didn't make it to the lists and charts and tops. But they did exist, they were broadcasted and maybe even released on VHS tapes — yet they weren't popular enough to withstand the test of time.
Years will go by, and no one would remember the titles from the ‘10s that we now call trash. Only the most remarkable hits would be remembered, the ones that changed or defined certain genres, ones that won awards, gripped way too many hearts, and left us speechless. All the other titles would be forgotten — just like the ones from the previous decades.
So no, the grass wasn't greener in general. But the grass that reached us is the most carefully preserved specimen of grass from that era. Now, continuing the metaphor, we're interacting with all the grass that surrounds us in this anime meadow, and alongside the nice and green herbs we may encounter wilted and unappealing ones. Just because we're not interacting only with a selected top that has been honed over the years.