the reason for limited OpCodes is to strangle the ability for the protocol to be changed in the future, other than what’s 100% necessary (such as Craig pointed out with updates to the hashing/cryptographic-algos as Moore’s Law makes old ones more at risk). This is why he mentioned the granite and marble banks use to convey stability. Without a stable money which cannot be altered in any way, it can be altered, and altered by financially motivated hackers (aka thieves). By limiting Script, you make it so the protocol is locked. Thus no adding Lightning Network for instance, bc LN is uneeded — larger block size takes care of mining fees (see WeatherSV & mempool deal for perfect example).
Imagine the rules of basketball were written on stone, and no one could change them. But the brilliant inventor left a little blank space on the bottom so maybe 2 things could be changed in the future. Maybe he foresaw that 2 point shots would someday need a longer shot — 3 pointers, or 4 pointers as basketball players became more skilled (Moore’s Law for basketball — like Steph Curry?). Maybe he wanted to account for humans getting progressively taller, so left a space for people to raise the hoop height? This would be reasonable. But if he left 10 extra blank tablets, people in the future could change the rules of basketball to something else entirely. With many many participants needing a stable money protocol to conduct business far into the future, allowing changes to the rules is detrimental — particularly money. This is why he points out gold not being the same if you add a neutron.
If you look up the history of Intel and the x86 processor, or better the history of the internet protocol, you can readily see how stability breeds growth and big businesses atop a rock of gibraltar foundation. Intel could have kept raising the bit count in the 70s and 80s, but their leadership smartly realized that upping the bit size was killing development, so they stabilized it and focused instead on making the 8-bit chips CHEAPER thus more attractive. This is exactly what BSV is doing. Cheaper transaction via bigger block sizes, attracts commerce.