Bt are "specific" in that they don't kill all insects, but they're still pretty broad-spectrum. Its not like one type of Bt kills only cabbage worms; the cabbage worm "specific" Bt also kills the larval stage of most types of beetles. I don't know how much biology you know, but Bt specificity is at the "family" taxonomic rank; e.g. a particular Bt will be lethal to all species/genera within a family, and often has reduced toxicity to similar families. This is one reason why an ant-specific Bt has not been released; such a Bt kills ants, but would also harm a number of pollinator species including bees and wasps (all of whom belong to close families to ants).
In GMO plants the toxin doesn't go away because it is continually produced by the cells - as far as the plant is concerned, its just another gene under a promoter (the thing that turns genes on and off) that is 'on' in cuticle tissues. You could think of it as equivalent to a gene that gives your skin colour - its always 'on' in your skin, and 'off' everywhere else. But in this case, instead of colour, its making an insect toxin in the plants "skin".
In terms of resistance, you have how resistance forms backwards. Transient exposures, such as those you get with spraying, are what cause resistance to evolve. The reason being that a portion of the insects are exposed to a non-lethal dose (e.g. due to being in a more poorly sprayed area, or because they move into the field a few days after spraying); this lets any insects with a small amount of resistance a chance to out-compete those without any resistance. The next time you spray you re-select for insects with some resistance, with more resistant insects out-competing less resistant ones. Repeat enough times and completely resistant species can emerge. GMO is always there, at toxic levels, making it much harder for resistance to evolve. Resistance can still evolve with GMO, which is why the more modern Bt strains express 2 or 3 different Bt toxins. This approach seems to be sufficient to prevent the evolution of resistance (its also why similar tri- or quadravalent-component treatments are used to treat things like HIV).
As to why non-GMO seeds are still propagated, there are a number of reasons:
- Most food crops are not GMO'd (only ~10 crops have GMO variants available) so there is a need to produce those seeds for producers.
- Crop breeding is still a major industry, and those strains represent their feed stock
- GMO is never done alone; it is always combined with conventional breeding approaches (which are pretty unnatural, btw) to produce plants with the desired characters. Think of it this way - with GMO you can add or remove a single select trait with high specificity. What you cannot do (yet, at least) is insert general desired characteristics.
- Those seeds represent a lot of genetic diversity which can be used in the future for breeding or GMO
- Some people just like old varieties.