On marathon speed, Philosophy costs 4000

. After bulbing that with my first great scientist, I can sometimes trade around and broker that for 20,000

total. Now, On marathon speed, 1 A.D. is turn 350. Discounting the first 100 turns spent researching bronze working and other worker techs, 20,000/250 = 80

/turn.

The AEsthetics brokering I probably pulled a bit earlier only adds to that.
My point is, the cottages in the measly 6 cities you were lucky to get up on Deity aren't gonna make much of an impact. Playing at that level requires that you streamline the rest of your game to focus on the game breaking mechanics that the level was de-facto-designed around. Perhaps waiting (by his estimates, anyway) 45 turns (135 marathon) to get strong cottages up is a bad investment, but that's not due to the intrinsic nature of cottages, but to the fact that investment itself has little value in that scenario.
Now, if you drop down to immortal, it's viable (for me at least). If you drop down to emperor, it works wonderfully. On emperor, when you tech AEsthetics, the AI will probably still be trying to grab Alphabet. The lower levels force you to actually do most of your won research. So if that's true, is it correct to tell people that cottages are useless unless you're a financial leader? The hierarchical mentality on these forums dictates that the lower levels aren't real levels, just a compromise till you get better. But assuming you accept deity as it's own thing, maxing your cities out with Hereditary Rule and getting up towns in the early A.D.'s is an art in and of itself.
I'm not saying that cottages are necessarily better than specialists, of course. But they have their own merit, and not just with financial leaders.