The sponsorship of pro cycling teams tends to go in cycles. Sponsors tend to sign up for 3 or 4 years, get their exposure, decide that they have achieved their aims and that continuing to pour money in isn't going to get them much extra and pull out. It's only the lottery people (FdJ, Lotto in Belgium and Holland) that have had really long term sponsorships. Last year CannondaleDrapac nearly went to the wall because a new sponsor pulled out (until EF Education First came in and bought them). Even QuickStep struggled to get a co-sponsor last year, so winning isn't a panacea.
Usually, teams tend to bounce from sponsor to sponsor, and retain similar characteristics (eg you can draw the lineage of QuickStep back to when they became Mapei-GB (and Lefevre came on board) in 1995, with a series of different names, but they have always been classics focussed). However, Sky have a massive budget, from one main sponsor. They are going to find it very difficult to get a similar level of funding from a different sponsor, so even if they get a new sponsor, they may have to cut back on the expensive riders that they employ. That might mean that they are less likely to win the grand tours, thus reducing the value they offer to a sponsor.
I can't help wondering what that means for people like Egan Bernal, who signed a 5 year contract with Sky a couple of months ago. Also, would G have stayed, if he knew that the team might be significantly weaker in 2020?