I'd have to agree - you'd need to be either an extremely specialist photographer with a very specific lens need that didn't happen to be available with Sony, or be at National Geographic levels where you actually need a selection of 3 dozen lenses at various focal lengths and speeds. Otherwise, I'd really find it hard to believe that 90% of all photographers in the world could find anything they needed in a Sony A-mount.
As mentioned, don't just look at lenses made by Sony. First off, be sure to add the very large backlog of Minolta lenses to your viewing selection. Then, include the nice selections available from Sigma, Tamron, and Tokina. Throw in a few specialty lenses by other manufacturers, and there's very little you'd miss between Sony and Nikon. Not that I know someone couldn't pull a lens out of Nikon's collection and point out that Sony's mount doesn't have that EXACT match...but if the difference is between an 85mm F2.8 macro and a 90mm F2.8 macro...I'd consider that splitting hairs a bit. Also consider there are a sizeable number of lenses available for Sony that don't exist in Canon or Nikon mounts either - like stabilized primes under 200mm (24, 28, 30, 35mm from F1.4 - F2.
or stabilized ultrawide zooms.
My lens collection comes from all over the A-mount world - and I haven't had a problem in getting a very diverse selection of lenses covering many needs - an ultrawide (10-24mm), low light primes (30mm F1.4, 50mm F1.7), midrange macro (90mm F2.
, ultrazoom (18-250mm), tele prime (300mm F4), big telephoto zoom (200-500mm). I've got 1 Sony branded lens, 2 Minolta lenses, 3 Tamron lenses, and a Sigma lens.
Check out places like KEH.com and B&H's used lens departments - not only plenty of lenses of all lengths, but pretty cheap too!
Sony DSLR-A580 / Sony 18-250mm / Minolta 50mm F1.7 / Sigma 30mm F1.4 / Tamron 10-24mm / Tamron 200-500mm / Tamron 90mm F2.8 macro / Minolta 300mm F4 APO
Sony NEX5N / 18-55mm F3.5-5.6 / 55-210mm F4-6.3 / Pentax K adapter / Konica K/AR adapter / bunches o' Konica & Pentax lenses!