My opinion and experience says they end for different reasons of course but the biggest IMHO is a hot young girl and an older guy, guess who moves on first> When she isnt with you, what is she doing? yep, what young girls do. Clubbing, keggers, dating etc Meeting guys her age, fucking them for free etc. After a bit aka, the money isnt as fast and free as it was, you are gone, left frustrated, maybe, heart broken, maybe but odds are it is you getting dumped not vice versa.
One other point, I now know several SB's who I did not involve myself with plus the ones I did. Rule of thumb, if she has you, she has several of you. The money is their motivation so more the merrier no matter what she promises. Given several or many "clients", what is the difference between that SB and an SP,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,the SP is way cheaper, less drama and you know the game in advance and have complete control
Just my thoughts