"it doesn’t seem like they make bikes like they used to anymore"
They certainly don't. The newest mountain bikes are far superior to bikes built 25 years ago. The state of the art has moved on. However, if you have a $300 budget you are not going to find much in a dual suspension bike. A new bike will be a department store special, and a used bike will have seen tons of use and abuse. I had a 1998 GT LTS, and it was a pretty good bike at the time, but I basically had to replace everything except the frame to keep it going. When I upgraded to a 10 year newer bike, I could not believe how much better mountain bikes had become. Yes, they don't make them like they used to, but you have to compare a new bike whose inflation adjusted price compares to the price of your vaunted GT RTS today. That would now be a $3000 at least bike