Testosterone is an immunosuppressant and leads to a shorter life. Simply cutting off the supply of T to a man can increase his lifespan enough to make up the general gap in lifespan between sexes. [1] There are even some theories about human mating preferences that suggest masculine faces are attractive as an indicator of good genetic quality because if you're able to be drenched in Testosterone and still survive you must otherwise have some above average qualities to make up the difference. [5] (Also recall how estrogen therapy or anti-androgens improved outcomes for male COVID-19 patients. [6])
The biggest risks that come from estrogen therapy are blood clots (estrogen thickens blood) and breast cancer. Blood clots are much less likely when injecting estrogen since it does not pass through the liver which would produce metabolites that complicate matters for your body. Further even though trans women have a much increased risk of breast cancer they are still 70% less likely to get breast cancer than cis women. [3] The risk of prostate cancer also drops by something like two orders of magnitude versus a cis man who has functioning gonads.
Taken all together it seems that if you account for things like the fact that trans women are one of the most impoverished groups globally and suffer from HIV at a rate of about 40% (as in 2/5 trans women have HIV probably because of doing survival sex work) you'd expect a well-off socially accepted trans woman on HRT to live longer than a genetically identical cis man. She has an improved immune system due to lower testosterone and her chances of prostate cancer have plummeted. [4]
[1] https://www.reuters.com/article/us-science-eunuchs/the-ultim...
[2] https://med.stanford.edu/news/all-news/2013/12/in-men-high-t....
[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8403717/#:~:tex....
[4] https://www.researchgate.net/publication/342512760_Prostate_...
[5] https://journals.sagepub.com/doi/10.1177/147470491301100508
[6] https://www.cidrap.umn.edu/news-perspective/2022/05/estrogen...