Bitcoin continues to outpace all altcoins no matter what they've promised. and that's despite Bitcoin's 1MB limit.
Because it hasn't become an issue yet.
But ultimately if Bitcoin isn't the best medium of exchange it won't be a store of value.
"Store of value" is an emergent property of a good medium of exchange. It's not something that can be separated as if it were independent.
I don't think the two are necessarily linked (gold, for s.o.v. without m.o.e. functionality, local currencies in high inflation regions for m.o.e. functionality without that of s.o.v.). However, it simply seems like an unnecessary risk to me to not provide that "service" (functioning as medium of exchange, scaled up to global demand) when the costs of a solution (sliding size) seem relatively low to me.