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The higher the abv, the higher the pressure on yeast, yes. But from what I can tell, at least by the time brewers were recording their recipes using hydrometers, beers up to around 10% were very possible.

For instance, Brew Your Own references some strong stout beers made in 1868 (recorded by a "G. Amsinck") here. (http://byo.com/bock/item/1623-when-stout-was-stout)

I plugged the OG (original specific gravity) and FG (final specific gravity) into a specific gravity to alcohol calculator, these are the results:

Dublin Stout: OG: 1.092, FG: 1.019 (ABV: 9.6%)
Treble Stout: OG: 1.096, FG: 1.031 (ABV: 8.0%)

This is not typical -- the Dublin stout is more like a strong export style. Ray Daniels in "Designing Great Beers" provides both more references from G. Amsinck as well as a table derived from "The American Handy-Book of the Brewing Malting and Auxillary Trades" (1908). For more common "porter", the ranges was more typically, say OG: 1.06 FG: 1.02 (about 5% roughly).

Ray Daniels' book likewise references historical Scotch ales similarly. At the high end, massive Scotch ales were present in the nineteenth century (with OG in the 1.120 range, FG in the 1.050 range, and an ABV of 9%). Also more common Scotch ale too.

It is true that by modern standards many of these beers would be relatively sweet, not as fermented.

I personally would guess that even without pure cultures, if you used a fairly simple and probably even known technique back then -- make a big beer, and put that big beer on top of the yeast cake of a finished small beer -- 10% is pretty doable. (The 20+% some craft brewers can go for, however, is probably not. :) )