"Old, proprietary software" describes something like 80% of all the software keeping the modern world going.

Some of it is new software. South Korean love of ActiveX for example, a technology dead for at least 10 years. Still getting new stuff written.

On the contrary, most critical software is plenty new - things like MS Office. Still bound to Windows.

The remaining systems rely on truly custom software and should be either airgapped (so no RDP) or rewritten. I'm thinking industrial - they should've planned for this many years beforehand. There were instances back when Windows XP was the main driver.