Wow, that just doesn't hold water in the world I've lived in the last 30 years. I know there are limits to what a stock speaker can handle, and you shouldn't put a 1000W amp to go with a speaker built to handle 30W.
But adding higher quality speakers, does NOT necessitate an amp, that's just BS to sell you more product. I've been around custom cars and sound systems since 1978, and what i've seen is the majority of what people call "systems" are just a crap load of mismatched parts. Most people don't look at signal to noise ratios, or anything important, they just think because the wattage number is higher it will be better.
A good two way speaker will make a stock radio sound better, it will not put out more power, and it will not DRAW more power. As long as the impedance is correct, there will be NO damage, period.
[emoji99]