I see where people are coming from when they say this, and no doubt at the moment English football is looking rosy, and celtic is not needed. But the good times in the English game aren't necessarily going to stay the same way as they are now- so opinions on letting celtic in the lower leagues could change in the future. I think for Celtic to have a chance of getting into the League structure then the English game would have to slowly lose its prestige like Italian football has over the years. In the same way that calciopoli brought an abrupt end to italian football arguably being the best league since the 80's: something could happen to England: Eg. if clubs continue to fail in europe or financial difficulties hit several clubs like Portsmouth.
Even then, its no reason to bring Celtic in. Nor have clubs really failed in Europe. It doesnt solve anything at anyway, it doesnt prevent other clubs from going down