ChanceDanger
Warlord
You said it yourself: when you work 4 goldmines @ size 7, you'll stagnate. That's not very hot, some riverside cottages would actually be better combined with some 1-2 goldmines as you'd still grow. Yes, you can grow on more grassland farms, but then you wouldn't be able to work the Gold in the meantime. Riverside cottages should be pretty much even in this case as you could work them constantly.
I'm not saying it is bad, but a) you're overrating commerce compared to food/production, b) it will still take some time to grow into all the gold times, not counting the time it will take to REgrow into them after you whipped for example.
Yes, so many gold hills are not bad, but you will still find it hard to work them constantly, and if you do, you give up on alot of whip potential. 2 or maybe even 3 gold hills are MORE than enough. What gold does is to speed up your earlygame, it will not carry you through the mid or lategame - actually, in the later stages of the games it's significantly worse than cottages, as it's food negative and your city will stagnate until you have Biology.
Why would you whip often, if at all, in an oxford city? Especially one which provides hammers with your commerce? I guess we could be comparing to a super crazy insane amazing start (every tile riverside and 7 gems or something of the sort where food isn't an issue at all) but even then you probably wouldn't want to whip that anyway. Just because the cottages sustain themselves doesn't mean they'll have the value of those gold mines for at least 100 turns, if not more.
Early advantage should (and usually will) trump how the city will look after all the cottages are given time to mature. At size 6 you have a choice between -1 food, 32 commerce and 15 hammers or growing to size 7 in 4(?) turns, 2 with granary (Didn't research, but 34 food total for pop growth at size 6 with +11 Food per turn). That doesn't sound like a slouch of the city to me. By the time that it would be considered a poor(er) city, you should have either settled a more balanced city that'll take away the temptation to throw Oxford there or taken by force. Fair enough if you play for the late game and care about the total 20 tile yield, but it doesn't detract from it being a very uncommon and excellent starting location.