I think the situation with Stroll is complicated much more by finances than with Hartley.
Clare Williams has gone on record about a week or 10 days ago saying they want to re-sign Lance for next year.
I think this has more to do with his dad's money, Williams could face a big drop in income next year if they don't get off the bottom steps. Martini sponsorship ends this year as well and not certain there is any replacement. Stroll's deep pockets may be their way to keep the lights on.
IMHO Williams are looking more shaky now than Sauber were a season or 2 ago.
Stroll isn't the strongest driver on the grid, but he's clearly one of the strongest starters, which is something to build off of, both in terms of his career, and to help Williams get exposure for sponsors. As such, keeping him around seems to make sense. Some are still cutting Max slack for being 'inexperienced', so Stroll should have that as well.
As for Williams vs Sauber, I don't know how parallel their situations are. Williams is an engineering company with interests outside of F1. They run the team like a business, so they're not spending money they don't have, but I don't know that the team is in danger of vanishing either. I believe Sauber is a racing team only, and they were being run (with all due respect to Peter) like a hobby more than a business. Now that they're owned by shareholders, the're being run like a business and not a sports team. Without non racing areas to fall back on, it would be easy to see Sauber closing up.
Both Williams and Sauber have trouble putting the right people in key positions. It's hard to get people who have the experience to take a risky position, and it's hard for people taking a risky position to get the experience!