Several factors in their favour are that most have spent their lives in fresh water - or at least a good portion of their lives there.
Another is that they were built to do rough work, usually in logging operations, and so were built to take abuse. They were also technically simple - little more than a hull/cabin with a very basic propulsion system.
Those who operate then now want to keep them going due to very high replacement costs.
A recent trip through Quebec turned up a few Russels in various places:
1. La Trenche was built in 1950. It is seen here working on a bridge project at St-François du Lac for Groupe Océan.
2. This unknown Russel is now a pleasure craft at Louiseville QC, July 8.
3. OC 34 ex Rapide des Coeurs was originally built in 1934, but extensively rebuilt in 2002. It is shown at Groupe Océan's maintenance yard in Quebec City.
4. Groupe Océan is by far the largest owner of Russel tugs, and has several in various states of repair at any given time. (The tug on the left is not a Russel). Second from left is H.E.Graham, built in 1964 - it appears to have had an engine fire. Third from left is Pascal D. ex Namekos, built in 1955. Far right is La Croche built in 1940. Quebec City July 6.
5. Le Phil D. ex Expanse, was built in 1961. It is seen here at Rivière du Loup, June 30 tending the dredge Ocean Basque 2. It too works for Groupe Océan.
The excellent Russel web site (although a few years out of date) is well worth a look, to see the huge numbers of boats the company built, and an explanation of their unique appearance.