If I do my homework math, it means you are making Sketch almost as expensive as Adobe Photoshop, right? (Adobe CC Photo Plan, which is Photoshop including Lightroom, is $9.99/mo.)
If I take into account the fact that many releases of Sketch are buggy as hell, for example the current one has problems rendering shadows when zooming (both 2015 MBP 15in & MBr 12in, both on internal screen & ext. EIZO), you are making purchasing licenses basically mandatory, since it might very easily happen the last version in “subscription” will be all lemon. Not nice.