@BlackAmericanoNoSugar
Mr Bennet isn't that old though, if he had Jane at around 30 then he's still only early 50s. There was nothing negative in the book about his health so he could potentially live to 70s or even 80s. So there could be a long wait for Mr Collins to get possession of Longbourn and he'll be wanting the best possible parish in the meantime. It's what Charlotte wants too, when she first realises that Mr Darcy might fancy Lizzie (when Lizzie is visiting Charlotte and he keeps calling to visit Lizzie) she does consider Mr Darcy's ability to gift a very nice living.
Well, we don't know how old he is, other than that he is presumably rather older than Mrs Bennet, as men tended not to marry until they could support a family and were usually older than their wives.
But even given the average life expectancy in England in, say, 1800 (c 40 -- though that obviously is massively skewed by infant mortality) that's not factoring in the chancier risks like epidemics, accidents and diseases then untreatable by medicine that made sudden death something Austen's period had to factor into their plans more than we do. I mean, one does see the point of Mrs Bennet's anxiety to marry her daughters before Mr Bennet dies and leaves them in reduced circumstances in which making a good match will be much harder.
Which is a long way round of saying that while Mr Collins might have decades of parishes, it wouldn't be that unlikely for him to inherit Longbourn quite suddenly, either.
For comparison, John and Fanny Dashwood, discussing whether to give the 40 year old Mrs Dashwood an annuity in Sense and Sensibility, say this:
“To be sure,” said she, “it is better than parting with fifteen hundred pounds at once. But, then, if Mrs. Dashwood should live fifteen years we shall be completely taken in.”
“Fifteen years! my dear Fanny; her life cannot be worth half that purchase.”
“Certainly not; but if you observe, people always live for ever when there is an annuity to be paid them; and she is very stout and healthy, and hardly forty."
I do entirely sympathise with Charlotte, though -- she's married purely so as not to spend the rest of her days a resented drain on her brothers' resources, so she understandably wants Mr Collins in the best possible financial position.