The best things in life are free!
This is true, and as such, I appreciate them much much more than something I have paid for.
If i have paid, there are expectations - something free, i am grateful for what is there, good and bad.
I don't find the science museum too crowded. I did do the NHM in half term once, on a rainy day - i wont be doing that again.
The should remain free. The museums are one of London's main attractions, if they start charging, fewer people will go, and as a result, fewer people will go to the surrounding area and spend their money. So from a purely economical point of view, yabu.
From the point of view that many people wouldnt be able to afford to go to these places at all YABVVU