BC doesn't matter (with a HUGE asterisk after it).
Yes, there are people who would LOVE to have BC.
Yes, there are people who will forego buying the system at launch because of lack of BC.
For the latter people, their sales will be replaced by other hardcore early adopters who aren't basing their decision on that. Some of those will consist of the former people.
For the former people, while they would love to have BC, it will either mean that they'll make the purchase anyway, or they will postpone the purchase until the new system itself has adequate software support.
All in all, the number of buyers in each of those two categories who simply won't purchase the system EVER as a result are very small.
And no, this isn't just for Sony. You want a non-Sony example? Wii U. The most successful system of this gen (Wii) and BC hasn't helped sell the Wii U at all. Now it could be pointed out that Wii software support was on life support by the time Wii U hit.. But let's look at the 3DS and the DS (I believe the DS is the most successful handheld of all time). MASSIVE support with the DS at the 3DS launch... yet 3DS sales were still SLOWER than DS sales (actual unit sales).
BC has traditionally been a sales tool.. it encourages transition to a very small degree.. but I think if MS, Sony and Nintendo learned anything this gen... it's that there isn't much reason to make it a focus. If you can sort of incorporate it, great (360, Vita). If you can completely include it, great (PS3, GBA->DS, DS->3DS, Wii->Wii U). If you remove it, it probably won't kill your sales (DS, PS3, and GBA all saw sales increase with new models with BC removed)
The early adopters who eschew the lack of BC will be replaced by other early adopters. The number of lost users over the system's life is practically inconsequential.