Perhaps the unsuccessfulness of CART is a matter of perspective.
1991:
http://www.champcarstats.com/year/1991.htm
You could power your car with:
Cosworth DFX
or
Cosworth FS
or
Ilmor "Chevy"
or
Alfa
or
Judd
or Buick
For chassis you could run a:
Lola
or
Penske
or
Truesports (yes, somebody other than Penske or Gurney would still make their own car)
'94:
http://www.champcarstats.com/year/1994.htm
Engines:
Cosworth XB
Ilmor C
Ilmor D
Chassis:
Lola
Penske
Reynard
Half the races that season, outside of the month of May, they sent teams home on Saturday. Too many teams for the pit space or space on track, indicating a lot of teams felt it was the place to be.
'96:
http://www.champcarstats.com/year/1996.htm
Engines:
Cosworth XB
Cosworth XD
Ilmor "Mercedes"
Honda
Toyota
Chassis:
Lola
Penske
Reynard
Eagle
From a motorhead perspective, it's hard to see CART as less successful than what we have now.
If CART was unsuccessful, I hope the future brings more of the sort of unsuccessfulness that CART had.