Obtained my BA in Psychology at age 31, Master's in Social Work at 36, second masters in Vocational Rehabilitation at 39 then took a long distance PhD while working full-time. Never used the PhD because I loved my job in Voc. Rehab so much. Highest salary was 62k. My DH with a BA in civil engineering made 84k. We retired 6 years ago.
Whose the PhD through? Long distance PhD's are generally a warning sign, but for some fields they are able to do hybrid programs these days.
What would it be a warning sign of? I know a few people who gained their PhD's through the Open University and they all seem to be very competent to me. I have to say that the Open University generally has a very good reputation in my country, they get high rankings. This might be different in other countries.
The only difference between 'regular' PhD's and distance learning PhD's is generally the type of students: instead of being in their 20s and living the high-stress grad student life, they were people who had worked in the profession for decades and were either retired or working parttime, and wanted to thoroughly research a pet project. Doing this through a formal program instead of just researching on their own 'for fun' gave them better access to resources like library facilities and people to discuss their work with. I know most universities offer similar PhD-programs for people not formally connected to the university.
I think there is a big Europe-US difference here. In the US, the Open University isn't a thing. I'd never even heard of it until I started chatting with a Welsh acquaintance who was taking OU classes, and I've heard of a LOT of universities worldwide. It's just not on our radar here. I agree that it seems to have a very good reputation and to be a good, rigorous school, and I'm sad we don't recognize it as a resource here.
Distance learning in the US is heavily associated with non-rigorous, un-or-non-standardly-accredited, for-profit scam universities. This doesn't mean that some universities don't offer distance classes -- they do. However, I've never actually heard of a distance PhD program in the US; the vast, vast majority are in-person. I'd be wary of someone who came to me looking for a job with a distance PhD, just as I'd be wary of someone who came to me looking for a job with a University of Phoenix degree. I'd want to see the abilities demonstrated more rigorously than I would for someone with a degree that I would trust. (I recognize that this is possibly shitty, but it is nonetheless 100% how I would go about it; if I can't trust the degree, I need the ability to be demonstrated more thoroughly than I would otherwise.)
Legitimate distance programs here would be more typical of a community college or small liberal arts college, and would often serve non-traditional students -- the type of students you mentioned above, except going for their AA/BA instead of the PhD. Notably, almost any school that offers a legitimate and reputable distance program offers programs in-person as well.
(Also, I should note that all of the above applies to academic fields, not professional fields. I'm not as familiar with graduate programs in law, social work, or medicine, and can't speak to them -- I wouldn't be surprised if a distance PhD in social work, for example, was much more likely to be a good, rigorous degree than a distance PhD in materials science. So many folks who go for professional degrees are already working in their field, and they need to be able to keep working while they go to school; it makes sense that good distance degree programs will develop there first.)