tonnage...
i don't think thats the case anymore - Tornado, F-15E etc.. were designed/built when to be effective warloads had to be at 8x 1000lb unguided bombs or CBU's or more - whats the warload of a GR4 now? 3x 500lb GBU and 3x Brimstone, and thats six targets, not one....
much as i'm a fan of GR4, and the idea of a rebuild with more powerful engines, more fuel etc.. appeals, i rather imagine that in cost terms it would actually be cheaper in the long run to buy more Typhoon/F-35, and that Tornado's basic shape is always going to be a massive radar reflector compared to the Typhoon or F-35, meaning that New Tornado is infact unuseable from its entry into service...