How to find the commons.

[steve B]

It takes a lot of looking. When the cards are actually hard to get it takes even longer.
If you want a specific card

Regularly searching Ebay.
Asking here.
Going to shows and asking dealers.

Being willing to pay a bit extra to finally add that one card.

Even some really common cards can take a while.
I didn't make an all out effort, but I needed Jose Canseco to finish my 88 Topps set. It had been a somewhat better card, so it was never in big lots I'd buy. But it had gotten "cold" in the market, slid to the dollar box, then to less. Usually the old no longer selling dollar box "out back"
Everyone was sure they had it. But even offering $2 for what was then a 25 cent card wasn't enough for anyone to go dig it out.
Yes, I could have bought a complete set for something like $10 but where's the fun in that?
It took around 2 years I think but I did finally find one.

[steve B]

I don't think any type of collector hurts the hobby.

Every type of collector affects the hobby in some way usually not by much. Sometimes by a lot. Any other collector my like or not like those effects, but they generally don't hurt the hobby overall.

Set collectors may tie up more difficult cards a player or team collector wants. But they also buy up commons that are slower selling, which makes those commons worth slightly more.

Player collectors or HOF collectors bump up prices on cards the set collector may need. But those higher prices slightly drag the commons up with them, also helping the set collector in the long run.

Investors, drive the prices of premium cards up by a LOT. That puts many cards any other sort of collector wants out of reach. But those high prices in turn make people finding vintage cards pause on the way to the trash, and that brings "new" stuff into the hobby. Which is good for everyone.

And so on for pretty much every kind of collector.