To get stunning UX design, upstream bugfixing and excellent marketing.
Let me explain myself. I love the levels of ergonomy and polish of Mac OS X. But it's closed-source software. If I use (and pay) Ubuntu, then great patches are sent upstream, which I can use in Debian on my servers and Arduinos. It becomes useful to everyone. With Mac OS X, we're not advancing the world. But when I used Ubuntu for work, I was impaired compared to my colleagues. Blame it on a lack of seniority, but a steep learning curve for my OS is the last thing I want at work. So no Ubuntu, no elementary.io, nothing that has rough edges.
What allows Apple to hire UX designers and do bugfixing is the revenue. Which in turns gets them a good marketing team, which persuades the world of adopting their software. Linux misses advocates towards B2B, B2C and B2Gov. If we want adoption, we need a stable income, to improve UX patterns, bugfixing and marketing.
The FSF says it's ok to sell open-source software, but you just can't prevent people from redistributing. So it's possible to design a system that requires a yearly fee to access the upgrade repositories. Of course hackers will find ways around, publish a torrent, or choose not to upgrade. But the majority of people want a system that "just works" and have money to put down for this service. Businesses, programmer shops, owners of Teslas and iMacs don't want to download their OS from an unsecure source: They want the top-of-the-art, official, upgraded releases.
My own threshold is €200 per year for my work computer. We pay that much of IntelliJ. The OS is the most important service in our stack, it deserves paid workers. My parents' threshold would probably be 50€/yr.
I think OSS volunteers will feel cheated at first sight, but the software should really push changes upstream and show the value in having a much bigger Linux community.
NB: For those curious, this comment has 18 points so far (10:43 GMT).