As there's already tooling, closing the 8Gb line results in a saving to Apple, both in terms of the RAM and unifying that RAM. In the case of the 16Gb an increased production results in significant cost savings, and all that is needed is to extend the run time which is precisely what they do for the 8Gb run as they don't know how many will be ordered and can just churn out more and the same applies with the 16Gb run, but economies of scale resulting in a cheaper per piece run, and cheaper RAM unified.
As yet I've not seen any evidence of RAM being upgraded on M series chips on the finished device by anyone albeit the RAM is nothing special in itself (to use the pun they are available and cheap as chips) and I seem to recall they are Hynix 128-bit LPDDR4X SDRAM, and at large scale purchase would I'm sure be negated by cost savings in the extra 16Gb run and eliminating the 8Gb run, but we have upgraded the SSD, which is doable and at a fraction of the price it would cost normally, but we can't let those out to clients as it would be outside of our duty of care to our customers and would not be covered by any Apple warranties and where doing that as a research and development experiment would not affect Apple sales.
I would though politely ask that you never request evidence i.e. from Apple employees or any schematics emanating from Apple to be posted, as it would likely involve the banning of a poster, and possible/likely action by Apple as employees would inevitably have signed an NDA and even third party distributors may be bound by an NDA.
it is suggested that a Chinese company has successfully upgraded both RAM and SSD, but I've not seen any compelling evidence of them updating the RAM yet, but it would not surprise me if they have upgraded both? We've not tried to update the RAM.
This link covers the upgrading of M1 RAM and SSD, but I don't consider it compelling evidence its been done successfully.