Unfortunately, you are far too right in your assessment of UA's problem of being too much to too many...
One only needs to take a look at the multitude of cabin configurations to see this... even before UA finished other reconfigs, they decided to reconfig again. There is a horrible inconsistency of planes out there floating around with 6 or so different interior decors (although the newer decors are very rare), some 74's STILL without First Suite, mutiple crew rest configs on the triple, the list never ends. UAL can't seem to decide what standard configs it wants on the inside.
The United fleet standardization plan was established more than 5 years ago under Greenwald, and it is exactly what is hapeening right now; albeit, the A32X have been merely trickling in.
It's hard to dump the 157 strong workhorses in the 737 classic fleet when most aren't even 15 years old-- hell, younger than a lot of MD-80s. The widebody fleet has consisted of more than 150 modern 747-400, 767, and 777 aircraft; it's the largest and one of the most modern long range/widebody fleets in the world. I don't see a problem with the way UA is deplying 75's, for it is utilizing them just like all the other majors.
As it stands right now, the only white elephants in the fleet are the 24 737-200s... The 727s are fast going and should be gone before two years' passing. 35 should be retired in 2001.
In conclusion, I think UA's fleet strategy is a mess interior-wise, but very soundly arranged type-wise. United will have 6 standard fleet types in less than two years: B747-400, B777, B767/757, A32X, B737Classic, and the token 737-200. The natural progression will be more A32X series orders, with a faster delivery shedule.
At least it's not like AA, with 7in the forseeable future: B777, B767/757, 737NG, MD8X, B717, F100, A300R. Hell, DL is standardizing to 8: B777, B767/757, MD-11, MD-90, MD-80, 737Classic, 737-200, 737NG. No retirement plans have been made regarding the MD-80, MD-90, and 737-200, except for the possibility of 737-700s replacing the latter for express.