I'm not seeing how longevity slows the number of generations you can have per unit of time. That seems like it would be determined by how quickly a species reproduces after being born...not how long they live.
Generally, the more individuals you have competing for resources, the more difficult it is to raise viable offspring. If there are far more elderly individuals who cannot reproduce, they are essentially preventing those resources from being used to mature offspring.
Yes, I thought about that but I went with the general assumption (based on life history theory) that longer lifespans equals longer times to reach sexual maturity.