You write that you've found birth and baptism records for children who have never been found thereafter. You ask, Would other researchers therefore assume these children did not survive past a few days, weeks, or months?
Isn't a basic rule of thumb: don't assume anything? Someone else once commented that the road to genealogy hell is paved with assumptions. If no one has found those children subsequently, then we don't know what happened.
We can't assume events, and that applies in any case, whether we are talking about a birth, baptism, or any other event.
If a set of parents have a child born and then have another child later and give both the exact same name, especially a first name-middle name combo, that is evidence that the first child likely died before the second child's birth. But it isn't proof. Not finding someone in records isn't proof they aren't there - it proves only that you didn't find them.
Depending on the culture, some families gave an honorary first name to more than one child (such as a grandfather's name) and then the child was called by a middle name, which differed from person to person.
I recently tried to help someone research a family and found a census 1900 with the family's father named Harry C. Surname and mother Eliza. Also found a census 1910 with the father named Charles Surname and mother Eliza. Same surname. Same kids (8) same ages same place same other info. This person then nailed down that finding as "Harry died and Eliza married his brother."
No no no - what happened is, we found a 1900 census and a 1910 census and the father in the family group was listed with a different first name in each. That's all. We don't know why until we find out why.
We could all use a reminder at times to let the records do the talking.
That's my take on it - and hope that applies to what you wanted to discuss.