Are you the EOL police now? If so, what is the point? Joomla 1.5.25 whatever version it ends at will also EOL this year, after years of development, probably thousands of commits, and thousands of extensions on the Joomla Directory.
It would be reasonable to think that the Joomla 1.5 series is more secure than any newer version of Joomla, as well as many less mature CMS systems. So if it's security that is your concern, I think you are misguided.
If the issue is some extra workload on your staff, then maybe you could create a new service level for a couple dollars a year which retains EOL software ("Legacy Support"?) or request donations or something else.
I don't look to Softaculous to make strategic decisions for me or protect me, whether from dead ends or EOL software. At the same time, you have Joomla 1.7 in your system, and 1.7.4 is called a Security Release, and you are not going to provide a Security Release? Is that smart? I would say "no".
Thanks. It's generally a good thing for the whole community, too, to keep pushing toward new versions. It's just that if we have production sites on 1.7.3, it seems like 1.7.4 is a safer bet for compatibility of templates and extensions than a version which has new features added.