What you're describing, just adding an extra segment with a minimum quantity, is quite common. Yeah, there's a discontinuity around the price change where by buying more seats you pay less money. Some companies just leave it at that (you can brag that you've got those extra paying customers, even though they might technically be going unused, so you do get at least something small out of it). Some customers are worried enough about the price that they'll run the math themselves and buy the additional seats to get a discount, others will just simply say, "I really only need 30 seats," ignore the math, and buy 30 seats.
Of course, a common solution is to charge the lower price for each additional seat beyond the limit. So the first 35 units sell for $50/seat, and the 36th sells for $40/seat.