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Nearly 20% of Microsoft SQL Servers running have passed end of support
(www.theregister.com)
This is a most excellent place for technology news and articles.
There you can see how bad they are treating their customers, declaring end of support against their wishes and demands.
I would say that this is a sign of a bad product. Apparently, compatibility between SQL server versions is not great.
I have never had a problem upgrading a SQL server. Granted, we aren't talking about anything fancy like database sharding, but the janky applications I work with have never complained.
Apparently, it is not only my oberservation, but the article says similarly:
However, I'm not a db admin and my perspective might be biased (infosec).
I don't know what they're talking about there, but that might just be ignorance on my part, because I'm not a database administrator. For the basic use cases, SQL hasn't changed in decades. For simple applications you could even change from MSSQL to MariaDB to postgres and make only minor changes.