they've legitimately never tried to do the opposite

I can't speak for marketing studies, but there are films that can be used to measure audience receptivity. here is a little discussed film called 'White Palace' which didn't set the box office on fire, which was nearly unique in featuring an attractive older woman with a younger man. I can't recall any others. Even the Thomas Crown Affair had only just rough age parity between the leads. Sadly the financial performance of films like 'White Palace' (apparently it was very good according to Siskell and Ebert, if I recall), may be a benchmark that is still used, discouraging similar efforts.

The Brosnan film may have lost some profit with a hollywood 'older' headlining actress (though it was a pillar of their advertising), unlike his previous and following James bond films which didn't and were about twice as big. This was when Brosnan was at the peak of his popularity, and could likely had more sway with studio decisions.

It's why you get copy cat films too, in case one studio hits a goldmine and cashes in on a underrepresented niche audience. Like mainstream movies appealing to Lesbians like Mulholland Drive or Bound, or gays with Brokeback Mountain, and stoners with Cheech and Chong and more recently Pineapple express. This was partly why independent films were a huge force 10-20 years ago, they were broadening cinematic horizens and catering to those niches and reaping rewards, but now they are largely integrated into mainstream filmaking.