As another comment noted, even if the stops aren’t unconditional the probability that no one will pull the cord for any particular stop on a crowded bus is near zero.
This assumes all stops are equally popular, but that's very unlikely.
I suspect a lot of bunching is caused by buses having to perform complicated manoeuvres around car traffic because of poor stop positioning. In Barcelona what many lines do is skip stops right before left-hand turns (the line isn't even listed at that stop). This way the bus can switch lanes well ahead of time and avoid having to wait for traffic to go though all lanes. This works because most lines stop every two blocks, so you never have to walk too far for the next stop.
But then it's a simple case of the front bus not letting anyone on (only opens it's rear doors). Everyone then gets on the 2nd bus. The people become spread out between the buses and the probability of not always needing to stop increases. That does obviously only work where your bus design allows it and also to some extent, your drivers being wise to it.
Controlling which doors open as dependent on how full the route is and how bunched the buses are is just going to confuse passengers and make for a horrible rider experience. I sympathize with the idea though, as veteran riders know to move to the back as the bus nears your stop.
I mentioned it as it's what sometimes happens in London when the bus is very crowded. They do it because at the point it makes sense the rider experience is already horrible. For most buses it is front doors for getting on and rear (middle) for off. It's already independently controlled so it is not extra complication. But I do appreciate other places don't have the same system.
And the passengers not being idiots who feel entitled to exit through whichever door is nearer their ultimate destination. Also the infuriated passengers who’ve been waiting for half an hour or more not taking bricks and bats to the driver’s window.