The two don't necessarily have to be mutually exclusive. Support subscriptions might still make more sense for enterprises. But an a la carte system allows individuals and small companies to influence prioritization and "vote with their wallets" without having to be able to afford a pricy enterprise support contract.
Support subscriptions could even include an annual/monthly/whatever number of credits to vote for issues, possibly at a discount compared to a la carte.
I think you're right. And that it'd take rather much time & inconvenience, for the customers, to in effect "micro manage" payments for individual features, and agree internally about if to spend money on this feature or that feature
At the same time, maybe for some rare & "really big" features, it could be worth the extra administrative work, to pay for that particular feature
Support subscriptions could even include an annual/monthly/whatever number of credits to vote for issues, possibly at a discount compared to a la carte.