Sure they can, in fact, any nodes that don't upgrade to a segwit friendly version will consider pure segwit txs to be an altcoin.
That would not make much sense. Please provide a source using the terminology "pure segwit txs" equal "an altcoin".
It isn't about terminology, it's about functionality. My non-segwit node doesn't understand anything about segwit. You may use it, but only miners and upgraded nodes understand anything about it. And if you want to pay a non-segwit address, you will not be using segwit. For further research see: Mircea Popescu (admittedly a loon), and his views on your altcoin.
Check your premises, running the qt client used to be the most common way to interact with the network, now it's not. SPV type interaction is well developed and widespread at this point.
Exactly who was talking about SPV? Nobody. So you're not denying that the resource usage has increased and thus the number of nodes keep going down.
I'm saying correlation =/= causation. SPV has served as an increasingly popular substitute for running a full node, which used to be the main way to use Bitcoin.