Do not think that all buy ups are as onerous as expressed above. Sometimes they can be a downright bargin.
I fly (unfortunately) a lot of short segments on AA (in the range of 40/year) so making plat is a difficult event. Last year I was short about 9000 miles and $1200.
I use the plat feature for same day change routinely and early boarding.
AA charged me $1200 to buy up at the end of the year. It was a no brainer. I didn't have to time to mileage run 9K miles and I am certain I would not have saved a lot of money if I did have the time (or desire) to do so. I earned far more at work than $1200 during the time I saved by not wasting it on a mileage run.
For me, the buy up made lots of sense, since I am now a plat doing the same puddle jumps with the need for same day change just about every trip I take. (And once in a blue moon they even up-grade me
)