I expect this to have a large downside in that it introduces a negotiable element into the price. And as such you'll see it become mandatory that you negotiate (just as frequent flier miles became mandatory) and as a result margins will decrease.
Airline yield management -- which includes both setting the price buckets for different fare classes, and deciding which flights/routes get allocated certain numbers of seats in different fare classes -- is a dark art.
And in case someone happening by is confused by that, I'm not talking about economy class versus first class here. In the US, for example, there may be a dozen or more different fare classes which all book the same type of seat in the same cabin of the same aircraft, but which have different prices attached to them. This is the source of a lot of (inaccurate) advice to do things like booking tickets on certain days of the week or at certain times to try to get the lowest price. Airlines dynamically allocate and re-allocate differently-priced fare classes to flights all the time, and unless you really really know what you're doing, have time (which isn't free) to spend on it and pay (which also isn't free, obviously) for a service that lets you scan inventory by fare class, you're not going to reliably get the lowest price.
For example, by my count currently American has 14 fare classes which initially book into the economy cabin (and with different rules about how and whether they upgrade to a different one), Delta has 15, and United has 17. These vary by level of discount, restrictions on change/refund, who they're offered to (Delta has some which are known to be explicitly a "we only use this to indicate a fare where we're trying to price-match another airline on the same route", for example, and all three have fares for things like government and military travel), etc. and price differently.