Is it really profit? I think that it's more mass affordability with a little profit tossed in. The less expensive you can make a lens, the more people can afford to get it. But less expensive also means poorer workmanship. It's hard to do both high quality workmanship and low cost at the same time.
I would have to disagree. Manufacturing these days is far more precise than it was even 10 years back and the new lenses coming out proves it. You can have cheaper and better, especially if a process becomes more automated and faster/cheaper to produce.
As for profit, take a look at sigma. they have been able to produce at least one lens of late that easily rivals, if not beats the Canon version that costs 50% more. (with more on the way that hopefully also equal the great results)