Kanger Subtank is extremely versatile and user friendly. It works with all kinds of coils, including Aspire BVC (regular, not Nautilus) coils for a nice, tight MTL vape. And FT sells all kinds of parts for it--original and otherwise--like spare glass tubes, acrylic domes, building decks etc. It's a good investment.
Smok yes, Kayfuns are RTA only--no drop in coils. I'm not a big fan of Smok atties, but YMMV.
I'm not sure I understand your question. The only coil that will be available forever is the one you build yourself--28 g Kanthal or stainless steel, 2.5 mm ID, works on all of my atomizers. I use my Kuro winder and make those coils all the time. Wire and cotton or rayon will never be banned. As for drop-in coils, who knows. I like my eGo One atomizer with CLR coils--those are drop-in rebuildable coils. They can be dry burned, rewicked many times, and they are ridiculosly easy to rebuild, so technically, those few boxes of CLR coils I own will probably outlive me.
And they are inexpensive--both the atties and the coils. I like the Mega eGo Ones, v1 or v2, 4 ml capacity.
eGo ONE tank series – CLR coil
Zenith is nice, but the coils are not rebuildable.