3 women in my extended family got pregnant naturally at 45+ and had healthy babies, so it is totally possible.
No one can or should judge, of course, but my observation in all those cases is that these mums (and dads - because they are also of a certain age!) just don't have a lot of energy to deal with young children or toddlers and they have in all cases found it very hard - while still loving and cherishing their little ones, it's a very different style of parenting. You have to be comfortable with not being able to give to your dcs a number of things that come easier when younger (physical evergy; sleep deprivation; patience; interest in peppa pig when young and Pokemon go when older; willingness to socialise with other parents; etc etc)
If I can offer my personal experience fwiw, i turned 40 this year and had a pregnancy scare (my eldest are 8 and 10) and totally freaked out - despite oohing and aaahing for years about having a third. But once it became a distinct possibility, I realised that no way I'm ready to go trough the whole hullabaloo again.
I am a little sad I won't have a 3rd but have realised 2 things: 1) I was really longing for the days my existing dcs were little, rather than an actual new baby and 2) the broodiness I had previously experienced was more a result of wanting to stay young, whatever that means. I loved my 30s with little ones, and was sad they were over. But for everything there is a season and I'm now really looking forward to my 40s with teenage children and hopefully my 50s of independence!
So in regards to your question: of corse YANBU, it's your decision. However, think carefully about the motivations behind it. Good luck!