Not that I’m a fan of Ubuntu here (I generally don’t run it when I can run anything else), but I do want to say I think you’ve missed the point of the Pro tier.
Ubuntu releases two stable versions a year which are supported for 2 years or so. This is like a slow rolling distribution, and makes the newest software’s available. It receives regular security updates from upstream, from Canonical, and from backports, again for up to about 2 years. Most users install this version.
Ubuntu LTS editions are similar to the above, but receive all the same security updates for 5 years instead of 2. These distributions are generally targeted for Enterprise users who value stability over having the newest software, and for whom upgrading comes with significant time, expense and risk. The 5 year window is customary among other distros, and is largely supported by and throughout the Dev community.
Ubuntu LTS Pro editions extend the LTS support editions for an additional 5 years, meaning a Pro distro enjoys 10 years of security updates from upstream, backports, and from Canonical where needed. Canonical might even open source their fixes back into upstream for other maintainers and distros to use, depending on the situation. However, since Canonical is providing the work, they charge subscription fees to cover their costs for it from their target audience: Enterprises who can’t or REALLY don’t want to upgrade
Why an Enterprise might not want to upgrade has to do with risk and compliance. Corporate IT security is a different world, where every bit of software may need to be reviewed, assessed, tested and signed-off upon. Major software upgrades would need to be recertified to mitigate risk and ensure compliance, which takes significant time and expense to complete in good faith. Not having to do it every 2 or 5 years is money in the bank, especially when the environment doesn’t introduce new requirements very often.
Canonical is meeting a market demand with their Pro tier by allowing these customers to spend a fraction of their recertification costs on a software subscription. It’s overall good for the ecosystem because you have what amounts to corporate sponsors pumping money into keeping older packages maintained for longer. This let’s them keep using the same software distro all the rest of us can use for free.
I’m not shy about calling bullshit on ANY distro that operates in bad faith, and they all get into some BS from time to time. Nevertheless, Canonical are acting in good faith on this, and are merely collecting money for their time and skill to provide maintenance on FOSS packages that might otherwise go unmaintained.
tl;dr: Pro tier is for Enterprise customers who need extra-long term support and are willing to pay for it. Canonical is meeting a market demand so they can remain competitive for use in those environments, which is good for everyone. It’s benign. Keep the pitchforks sharp and the torches dry for another day.