I'm one of those that have said for quite a while there will be a revision for the Switch and my guess would be something like September next year just in time for the build up to Christmas 2018.
Reasons I would give;
A. Most importantly the current Switch isn't very good, its slightly too big and has short battery life. It's not an optimised design for portable use.
B. The Tegra chip in it is the old 20nm version which likely will no longer be produced. Nintendo may be forced to redesign to a 14nm fabrication process or better and so if they are redesigning the main motherboard may as well make other revisions. This is the pretty much normal for consoles, I think ps3 started at 90nm and was down to 40nm or less at the end.
C. It's claimed they were struggling to get some components due to Apple and a shortage of flash memory and companies were having to pay more to secure stock. A common solution to this is to redesign to use components with better prices and availability.
D. This one is more debatable but if there is a slowdown in sales then often providing a updated product helps with this. Nintendo DS did good business but DS lite was much better and sold much more. It feels to me that the current Switch is the Nintendo DS but a much improved DS lite style update is what will really push Switch sales.
It's difficult to judge because currently there is still stock shortages were Switch is popular so we don't know the true baseline of demand for Switch worldwide. I suggested there were 6 million core fans that needed to be satisfied before there was a drop off in demand and whether I'm right or wrong we aren't at 6 million yet. I thought we would be by now but stock shortages has prevented that.
E. The simple fact is there is more profit in a smaller sku, both in manufacturing costs and logistics assuming it will be the same performance and using slightly later improved fabrication. Smaller cheaper screen, lower capacity battery, cheaper motherboard, smaller heatsink/fan or no fan at all etc. Might get more in a shipping container, could be 10,000 instead of 8,000 or whatever the real figure is, suddenly shipping per unit is significantly less.
F. In the redesign they address all the reliability and return issues they can to make a more reliable and durable product. This means more profit again for Nintendo. Re-designed dock etc.
G. Lastly its what Nintendo do, they have a strong track record of releasing updated versions of consoles on such a timeline. Sometimes the revision is more hidden, massive internal design changes and sometimes its a new model but its pretty common for them to make such an update. Even if Nintendo don't do a major redesign of the exterior they may do a major internal revision using improved fabrication which means longer battery life and other improvements.