I really struggle to see how this will help with merch selling.
As it stands, the teams churn out new stuff every season. Obviously they will want to keep doing this, they aren't going to want a three year old Mclaren cap still being current, so why does a fixed number help them? Designing new team wear each year isn't going to be made cheaper by keeping a number the same, nor are the drivers themselves going to want to bring out, say a t shirt range but not refresh it every now and then.
I suppose the only option would be for the bigger names to establish a kind of brand, as Cristiano Ronaldo has his 'CR9' insignia, we might see Lewis Hamilton having an 'LH3' or something like that, but that's about the only thing I can think of that a fixed number brings that the current system doesn't.
I quite like the numbers reflecting the success angle, but so be it. Just don't see how the prime reason they seem to be giving is actually a reason at all!