As you have already pointed out there is no right answer to this but here is how I deal with WP sites after they are done.
When building a WP site for a client, through the whole process we make them aware of our aftersales support.
Our support has different levels with the levels giving better SLA’s than the last one. The client can purchase so many hours of a programmers time at a rate agreed by both. These hours can roll over but not into next years contract.
Many of our clients purchase 2 hours a month which is ample time to take care of upgrades and testing. As part of the contract we also install monitoring plugins to make sure we are notified of updates that need doing as well as any wrong doing on the site. The user account we hand over is also the Editor account as they have no need for Admin access. This stops the client from playing with things they shouldn’t be. However if they want admin access that can be granted but with a warning that if something breaks because of that then they may have to purchase additional hours to fix.
The great thing with a support contract is that you get some regular guaranteed income. Much of the time the hours are not used so you are getting paid for very little work. The client gets an up-to-date website which helps keep it secure. Any additional big project work is more likely to come back to you as you are supporting them rather than coding and leaving them.
Now I’m part of a company that does this and I know you are looking for a freelancers view but I know a few freelancers that have a similar setup.