Yes, you could continue using the old unmaintained app, but this is similar to using old proprietary app versions that lack security updates and are always at risk of stopping to work due to some changes in your OS. So that is far from ideal.

Non-commercial is really not well defined legally. For example in Germany, a public tax funded broadcaster was found in breach of a CC-BY-NC license for using an image on their website. And many similar legal examples exist. So basically anything that involves a service offered to more than one person, even if totally free and donation funded, is not safe from litigation.

And obviously, if upstream changes the license to something that triggers a hostile fork, it is unlikely that you will get a commercial license for that hostile fork. Furthermore, even if you somehow can make a deal, you will always remain hostage of that proprietary license.

FOSS licenses are explicitly designed to protect the users of the software from such potentially abusive licensing, so I really don't think anyone will see this as an improvement.