The article doesn't make any argument why the current situation is unsustainable - it does point out some problems, but as far as we can see, there's no reason why current situation, with all its drawbacks and limitations, couldn't go on for ever, i.e. be sustainably sustained.
Some contributors will quit, but that doesn't imply that the system is unsustainable, as long as some more people will temporarily make some contributions, and we're not seeing an imminent collapse where people will stop scratching their itches and publishing the resulting code as open source.
Your web site said “[CLAPPING HANDS SIGN] 3 claps”. Since I didn’t know what that meant, I clicked on it. It changed to say “[CLAPPING HANDS SIGN] 4 claps”. Now I feel like the person in that old joke about someone standing by a well or a cliff, repeating a number out loud.
Some contributors will quit, but that doesn't imply that the system is unsustainable, as long as some more people will temporarily make some contributions, and we're not seeing an imminent collapse where people will stop scratching their itches and publishing the resulting code as open source.