All but the last requirement are completely reasonable, but software vendors are more afraid of being left behind or exploited than screwing up their user's wor

All but the last requirement are completely reasonable, but software vendors are more afraid of being left behind or exploited than screwing up their user's workflow. They're not usually updating to keep their current users. They're updating to gain new users. And if it's free software? Forget it. You're not the customer. The last one is unreasonable because it's impossible to know what third parties are going to do. If they use the plugin API in an undocumented way, then they're the ones being irresponsible.