Rides are relatively expensive to run but compare DBGT to any other ride on park, for staffing maintenance, parts, etc. They gave themselves an enormous slippery slope there. Merlin/Thorpe are definitely not thinking about operational costs when they make these decisions, so I don't think that's any reason why old rides are closed.
More usually rides means more theoretical capacity for your park, so you can grow your park and get more people through the door, so more money! That's the usual model anyway. Thorpe have plenty of other space to expand so there's not much case for removing old rides. That certainly wasn't the reason Loggers Leap was closed.
So yes you can keep adding new rides to a reasonable extent. As long as you're not adding DBGT, and that you're maintaining your old rides well enough to avoid a big gridlock situation where it now costs a big lump sum just to keep an old ride open. Slammer was a bit different though, it always was unreasonable even with maintenance.