FN P90
Colt Model 1911 beats the FN P90 as a PDW.
If you want something that you can aim like a rifle, a US Model M1 Carbine works fine (about $400 to $700). If you really feel a need for select-fire, pay 10x more (about $4000 to $7000) and get an M2 Carbine.
The requirement specification for the FN P90 was for a lightweight, low-power weapon that has more firepower than a handgun, that can be issued to non-infantry personnel in the combat area -- people like truck drivers, armored vehicle crewmen, artillery crewmen, etc.
The M1 carbine was designed for exactly the same application. It is almost as light as a FN P90. It has been made in vast quantities, so the bugs have been worked out (more reliable). And if you need full-auto, M2s are available.
For some real de-ja-vu, take a look at the 5.7mm cartridge of the P90. Have you ever heard of the Johnson Spitfire round? It was a .30 Carbine cartridge necked down to 5.7mm. You can find these rifles available, but the ammo is hard to come by (re-chambering a M1 carbine for the FN 5.7mm round -- that's an idea!).
With military weapons, it's not necessary to be a creative genius to make money. Just dig up the history books, redesign it in plastic and pay off some gun writers.