In developing countries with low wage levels where the labour intensiveness in the operation and maintenance of steam locomotives is less of a factor maybe, but not in high wage countries, where having to pay a fireman (would the term have been changed to fireperson due to political correctness by now?) and having to pay them well since it's dirty and hard physical work, for each locomotive and the costs for all the other installations neccessary to operate steam locomotives like watering towers, coaling stations and railway turntables would soon outweigh the costs of electrification
As MattII points out, you can fire steam engines with oil (or, for that matter, I should imagine, any fluid burning substance, eg. powdered coal or natural gas), which means no dirty shoveling; in fact, a number of American steam engines in the late period were fired with oil, such as Southern Pacific's cab-forwards designs (necessary because the tender couldn't lead the train). There are also surely technological and design changes that can alleviate some of these concerns--for instance, would it not be possible to design a cab-forwards
hood unit-like locomotive? Then there would be no need for railway turntables at all.
More to the point, I only said that the era of steam would undoubtedly last longer. I mean, can you
really see US railroads electrifying the
whole country, including places like, say, branch lines in remote areas like Nebraska or Utah, as quickly as they introduced diesel engines? Especially since now they not only have to continue maintaining the old infrastructure as they introduce electrification, but also have to pay for maintaining the
new infrastructure, which is not going to be cheap, either? While the railroads might be on a firmer financial footing ITTL because of the absence of air travel and possibly long-distance road transport, it seems improbable that they are going to be enormously stronger; and the railroads did suffer significant financial issues during the 1940s through 1970s that would cause problems given the aforementioned massive costs involved in having to electrify while still running steam in non-electrified areas. Assuming, of course, that World War II still happens, which obviously isn't a given...
That being said, the only places I can realistically see steam surviving to the present are, as you say, third-world countries where the low cost of labor removes most of the disadvantages of steam to begin with and the inability to finance the necessary electrical and rail infrastructure would prevent electrification anyways, and
possibly some little-used, remote routes in more developed countries where the cost of electrification is great enough and the benefits small enough that despite perhaps higher ongoing costs it doesn't make any sense to switch to electric (that is, where the net present value of the upgrade is smaller than the cost of upgrading, because of the time cost of money).