As someone who has been collecting since the beginning, no theme that has been extremely popular has remained extremely popular. Eventually, things get too hot, and new stuff turns people's heads. So the hot things become yesterday's news. Any of the other old timer's could tell you about pins they never thought they'd own, that have made their way into the collection eventually. So don't be discouraged about starting something that is so popular. But a word of caution, figure out what price you are comfortable with, and don't stretch too far, thinking the price will only ever go up. Five years from now, what you paid $75 may only go for $30. If it's a pin you absolutely adore, that may still be okay, because the enjoyment you received from the extra years of ownership might be worth it.
IMO, the PTD's have been a strange phenomenon, because the pins themselves remind me more of your basic open edition pin, or even Hidden Mickey pin. Since they are essentially freebie pins, they are simple designs, usually small, and feature limited colors. There's nothing wrong with that, I collect all other PWP or GWP pins but only these have generated "frenzy" levels. The use of so many rare characters, and smaller LE sizes somewhat explain it, but I still tend to fall on the side that the reason the pins go for as much as they do, is because so many people bought them, not to keep, but to sell or trade. The tail is chasing the dog, so to speak. So long term, there aren't enough real collectors to maintain the high prices. Eventually, trade and restocking stock has to be liquidated. This isn't meant as discouragement, as when the "next big thing" comes around, I plan to actively accumulate more PTDs. Just to encourage prudence. Buy/trade for what is easily accessible and your favorites, and time should help with the rest (either prices come down, or as you get older and find better jobs, you have more money)