No they were not a bad idea.
The 300D has been out what, 12 years? And we now have an 'affordable' full frame camera. A bit more costly than the 300D entry price in real terms. And what you can buy a 1100D with EF-s lens for a sixth of the price.
Folk who wanted better glass initially bought off brand (UWA's, macros etc), or bought EF.
I never bought an expensive EF-s lens mainly because there were better third party alternatives.
The whole DX crop thing is dead handy, but in this megapixel obsessed age do you really want to downgrade your nice 36MP to a 15MP? 24MP to a 10MP?, or your 12MP to a 5MP?
I'm happy with APS-C and will continue to be happy with it. My cameras are working well enough and giving me the same results they always have, which I was happy with at the time, and am happy with now.
The next EOS I'll be buying will be 2x EOS c, probably the C100 or equivalent. Which is... super 35! Which will work fine with my APS-C format Sigma DC and Tokina D lenses in EF mount. I'm a bit different, I use my DSLRs for work (video) and frankly if they are working then I don't mess with them. If I change one camera I need to change them both (my t3i and 7D are matched iso for iso, same processor, so intercutting cameras is not an issue, would the same be true of a digic V and digic IV?)
But the cameras will work, may even be hired out, they'll pay for their keep. I'll use my DSLRs for stills, as that is my hobby. But I would never have bought a 7D if it wasn't going to work. I would never have bought a f2.8L telezoom if it wasn't going to work. I don't have the need for full frame, and thanks to the c cameras I don't think I ever now will.
Canon more than any other company has opened up DSLRs to the masses. Canon more than any other company has pushed expectations at the lower end of the market, giving people better cameras for less money. The folk who want better will always buy better, upgrade, or enter the market at a higher point.
EF-s let Canon sell shed loads of cameras, let folk use EF lenses on them, and give them an upgrade path.
And still do.
If the next rebel is FF and cost less than $1000, then yeah, long term EF-s might be on the wane, but that won't write off the successes of the EF-s mount. And who knows, future FF EOS cameras may not even have mirrors to strike, if EOS DSLRs can tell a flashgun that a lens is cropped, then the camera already knows...
If my T3i can put on stupid square crops then a future mirrorless 5D or 1D series can put on APS-C crops when and if required.
I remember the olden days when folk were more obsessed with photography and getting the right shot than gear. It seems to me that the immediacy and speed of learning on a DSLR has put a lot of folks noses out of joint. So they have to diss the cheaper gear. Have to have only L lenses. Have to have the latest most expensive camera. Yet newbies with an eye, or dedicated learners on a budget are doing more interesting work. Must be infuriating.
EF-s lenses have a bit to run yet, so long as students, beginners, occasional photographers want a good camera for less than a months wages, lang may their lum reek!