>he quits or dies you hire another one.

And in the weeks/months until you found one ?

It's not like babies appear over night, you have months to prepare.

As has been quite clearly stated elsewhere, hiring a dba who wants a gig for 6-12 months rather than an indefinite gig is either much harder or much more expensive than hiring a dba on the indefinite plan. Same reason consultants get paid a lot more; the person you hire knows they're going to be job searching shortly.

There's often specialist work than can be delayed or performed more slowly by other team members. I'm not sure how that's relevant.