T460s only has partially replaceable memory, some of it is soldered on. And god help you if that fails outside warranty, spare Thinkpad mainboards usually cost more than a new laptop.
I'm curious about the mother board comment. I have a t430, and rebuilt it the motherboard cost me 30$ via ebay. Searching for the t430s I'm seeing about 40 -> 80 on ebay. I'd think that by the time you need to replace the mainboard, it'd be at more attainable levels on the second hand market.
Third party mainboards are either used (and you don't know whether and for how long they'll work) or not initialized (throwing errors during boot). Factory replacements via Lenovo's spare parts service cost you $400+ for simple models and $800+ for dedicated graphics models.
RAM is pretty reliable so I'm not really worried. One thing I just realized though is that 20GB is probably the absolute max for this machine. Since there are 4GB on the board and the chipset can only do 32GB it's pretty likely that 4+16GB will end up being the maximum.
I've opened the machine and it's a shame they don't just give us 2 DIMM slots and 2 (or 4) M.2 slots so you can just splurge on some more RAM+SSD goodness.
If the T460 had the same display options available, it would be the perfect alternative: Two DIMM slots, two mass storage slots, two batteries, and still quite compact.
It will, but you'll void your warranty, and it's annoying that Lenovo doesn't offer it themselves – all their notebooks are built to order anyway, so they're actively hurting themselves by artificially constraining their customers.
The T460p has fully expandable RAM (almost 5 pounds), but it's a lot bulkier than the other variants (but somehow has smaller batteries), and for inexplicable reasons uses inferior materials. (Pure ABS plastic like on L/E series Thinkpads, not fibre-reinforced.)
Lenovo's model policy is somewhere between "annoying" and "fucking insane", every laptop is seemingly made by a different team and none ever talks to each other.
The '-s' models are all designed to be slimmer counterparts to the non-'-s' models. Apart from that, the T460 should be identical to the T460s; and I'd recommend the former instead of the latter for the same reason you mention.