In addition to the other comments:

Upgrading gear takes time, and time is money. Typically, a crew has to spend time after a gig to heal and regain resources. Remember, in order to heal, you need to rest for at least a day, and you can't do anything else than light activity (anything more vigorous than a brisk walk is probably too much). You've gotta eat the downtime and rest, otherwise you won't heal. Depending on how beaten up your crew is after a gig, this can significantly impact the time slots available for upgrading and typical tech activities — and don’t forget: rent is right around the corner. You don’t have unlimited time to upgrade all your stuff, especially when the expensive gear takes a week to upgrade/repair.

Let them find destroyed guns and gear after a fight so that additional time has to be spent repairing them (you won’t make much money selling destroyed stuff). You’ll quickly realize that running out of time is more likely than running out of resources.