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Another way to think of canon information:
If almost all Slytherins are purebloods and there are quite a few pureblood families whose members consistently Sort to other Houses then the proportion of purebloods in Harry's year has to be higher than 25%, and more importantly, the proportion of purebloods among the wizarding-born has to be higher than 33%. If so Ron's claim that without marrying Muggles wizards would have gone extinct and even more so Hermione's claim about there not being enough purebloods to fill the ranks of the DEs have to be unfounded.
If almost all Slytherins are purebloods and there are quite a few pureblood families whose members consistently Sort to other Houses then the proportion of purebloods in Harry's year has to be higher than 25%, and more importantly, the proportion of purebloods among the wizarding-born has to be higher than 33%. If so Ron's claim that without marrying Muggles wizards would have gone extinct and even more so Hermione's claim about there not being enough purebloods to fill the ranks of the DEs have to be unfounded.