On the very rare occasions I tour I use two panniers, not full by any means but the engineer in me sees it as balanced. However to cut down on weight I am thinking of taking just one, I see many riders with just one and I wonder does it make enough difference to balance to notice?
I mount and dismount on the nearside and so propose having the pannier on this side too. Am I overthinking this?
Can't really argue with the most of the points raised and I did a 40km commute with one pannier.
One pannier to lug about at the end of the day is easier than two, although go abroad and the pannier on the outside no longer protects the drivetrain.
However, it's Touring we're talking about and, for me, there's always a certain mystery to Touring and that can well include road surfaces. Or sometimes just surfaces. If you're prone to going onto some rough stuff that's when you might notice some handling issues. Steep, loose gravel on an unknown road? I'd appreciate the balance.
Setting weight aside there's another aspect to consider - dryness. With two or four Panniers I always have dedicated "Dry" ones - the ones that do not need to be opened during the day.
As always, a lot depends on what kind of Touring you enjoy. Someone on a camping trip might appreciate the "dryness", someone staying in a hotel with twisty, steep stairs might appreciate one pannier over two.
Then there's always the Bikepacking setup to consider.