A lot of small businesses use Qt for free under the LGPL license (allowing them to keep their application closed-source as long as the user is free to replace the Qt dynamic libraries). For the past several years many new modules are commercial or GPL only, preventing their use by LGPL users. For example, QtWayland, QML Virtual Keyboard, WebGL, Qt Quick 3D.
Some companies would bite the bullet and pay for Qt their exorbitant fee, then let the license expire and keep using the old version. As you say, this will no longer be possible.
I find Qt 6's license policy awful, I simply don't believe in "pay us forever even if you're happy with an old version". Early on Qt 6 even had a draconian commercial policy that wouldn't allow you to distribute an old binary after if your license was expired and you stopped new development, thankfully they walked this back.