I've always sort of wondered ir the D&RG kept some of the lighter engines(4-4-0's, 2-6-0's, 0-6-0T's) around after 1890, even though they weren't needed and certainly were not after 1893, in hopes of selling them off. A lot of them were eventually sold before 1903. The UPD&G/DL&G kept a lot of surplus engines around during the 90's too (brooks moguls, 1880 baldwin consols) which C&S managed to sell in short order in the same time frame.
As to the Class 56 engines, such as #71 and many of the others in that dead line, I've always suspected they were gradually pushed out of use in the first decade of the 20th century by:
1) The arrival (1903) of the class 125 engine
2) The conversion of the Montrose - Grand Junction line & North Fork branch to std gauge (1906)
3) The "Rich Man's Panic" of 1907, which I have seen some authors state hit the Colorado mining industry at least as hard as the Silver Panic of 1893.
After 1908 there weren't any of the lighter engines around and only a handful of Class 56 left, most of which were converted to 0-8-0 for switching use.
When the new boiler law took effect in the teens, the few survivors of class 56 and of class 45 1/2 were judged not worth putting new boilers on and just plain surplus to needs.
At least that's the way I've always interpreted the data. I could be way off base though.
Hank
PS In a similar fashion, a lot of the C-16 engines not scrapped right after the K-36's arrived were held on the Alamosa deadline for years, many of them before the Crash of '29, just in case.