I wouldn't characterize them a definitively lost customer, but I think it's realistic to assume that a substantial proportion of PF buyers aren't 4e customers and some of them are so because of the presence of PF. I think it's also reasonable to assume, whatever the exact percentage of customers lost or now shared is, PF has made substantial inroads into the pre-4e WotC customer base at 4e's expense.
Whether or not the customer is completely lost, however, that's money spent that 4e failed to attract. Each customer is only spending a particular buck once. So whether or not an individual is a customer of both (or many other) games, they are in competition.
Yeah, I don't think we can draw ANY conclusions. For example:
I never bought ANY 3e or 3.5 products at all. I tend to buy most of the 4e books. I haven't bought any PF books.
Sonny buys PF and 4e books, I'll assume he bought 3.5 books too.
My sister bought 3.5 books, bought 1 4e book, and hasn't bought any PF books.
Someone somewhere bought 3.5 books and now only buys PF books.
Some other guy somewhere never bought 3.5 books and now buys PF books.
Now, if we knew how many people fell into these different categories we'd have some idea of what's going on. Lets assume we can rely on the PF and 4e sell about the same that tends to come from what market data we have. In theory EVERY one of those PF customers could be people that weren't buying 3.5. Realistically not, but the possibilities range all the way from PF hasn't hurt 4e one bit, to PF took away half of 4e's cookies. We don't even know relatively how much 4e sells vs how much 3.5 sold in its last year. WotC might in theory be perfectly happy if 4e simply increased sales enough above the decay curve of 3.5 to pay for its development. Note that no reference there need even be made to PF, and you'll note that Mearls sounded almost puzzled by questions about competition from PF, though clearly he realizes it sells well.
Yes, it goes without saying that WotC will look at Paizo's sales and think about how to get that business. That doesn't mean they are desperate or think 4e somehow 'failed'. Nor does any of that indicate that the intelligent response to Paizo would be to abandon the half of the customers you've got and go after the other half that the other guy has.
And lets not kid ourselves here people. While we have probably all played older editions of various games because that's what we had, or there was some specific reason for it, nobody really wants the current version of a game they like to become the past version. WotC DEFINITELY doesn't want that, because if they release some 5e I don't like, I won't buy it. They don't want that. Any 5e that comes out WILL have to please the 4e (IE actual paying customers today) before all else. Don't kid yourselves about that.