1.00000000

=

1.000000000

This is true.

But in our scenario one Satoshi isn't the 8th decimal place anymore.

1.000000001

That is a new Satoshi that didn't exist before and cannot exist unless consensus changes that one day.

So you have to ask yourself how many satoshis make up 1BTC in this scenario?

1BTC = 1BTC

This is true, because to accumulate 1.0....

You need 100M of the current lowest denominator, which if we're adding decimals doesn't change the amount of Satoshis as we describe them today,

That's why LN uses the millisat denomination for everything after 8 decimals so as to not break consensus on how we describe "1 Satoshi".

So if we do add decimal places - we should probably also change the name of the smallest denominator to millisat - which doesn't change anything but can give people a marker of how to count to 1BTC.

"I have 100m Satoshis + 1 millisat"

= 1.000000001