lol :-D indeed auto is not good at that. 
On your question of sill strength, they are indeed quite a main part of body structure. If your welding is half decent & internal structure not too heavily corroded then they probably still suitably rigid enough. 
I'm not a fan of rust nor spending much money on repairs of corrosion. 
As you done it yourself it was a cost effective option, just keep an eye on it & give inspection for corrosion annually before MOTs & see how it goes, you may find rot spreads quickly (18 months/2 yrs) aound joins of new patches or spreads out to suspension mounting areas on the rear. If corrosion progress is very slow you may see 4/5 yrs before major issue again. 
Main thing is keep cars running costs/options in line with this & not heavily invest in it in other areas.