Less death of culture, more homogenization.
These are one and the same.
Culture is multifaceted anyway. You can destroy the racist part of the southeast US's culture without affecting the rest of it. And I don't think an even larger country / world wide government would really necessitate or even have much impact on local cultures. Changes in law would matter, of course, but off the top of my head the biggest impacts I can see there would be human rights related. I would expect a single world government to mandate and enforce basic human rights (like for women) that aren't currently respected in many parts of the world, but it's not like big brother is going to get rid of NASCAR and mustard based BBQ. Or bagpipes. Or whatever.
In the end laws are the foremost expressions of culture.
And don't kid yourself into thinking that only bad and unjust laws would be the ones to go. Just take the EU for example and how they are constantly nagging at Swedish state opacity rights - those wouldn't last two seconds in a One Europe Order, let alone a world union.
On a further scale of criticism against a world state is that it wouldn't function as a democratically. Democracies function best the smaller they are. The closer to the voter power lays, the more power the voter has to hold the power accountable. Democracy functions best when people are culturally united with common thoughts, values, and customs, not to mention geographic placement. There is no rational reason a bunch of Frenchmen should get to rule over Sweden and Swedish people and resources, but in a One Europe Order they would just by sheer mass of their populace.
The above is even more important when it comes to welfare states. They require a strong, united core, and trust and loyalty between people otherwise people are not going to be okay with making sacrifices to ease the troubles of strangers. This is actually the main reason I think the US will never develop a functional welfare system - Americans simply seem to functionally lack any ability (or perhaps it's more a matter of willingness) to solidaritate with their fellow Americans on an impersonal level, and it is far too big for the on-the-personal-level otherwise generous culture to be enough. Similarly, the Scandinavian welfare states so often held up as an example to follow by American liberals only managed to succeed the way they did because of the strong nationalist sentiment of the times - that's what bound people together and have them a sense of common purpose and community - and as nationalist sentiment has weakened, so has the functionality of the welfare state. People are less trusting, less willing to work for each other, and more willing to abuse the system for their own gain.
Speaking of which - in a global welfare state there'd still be the question of differing work ethics, but on the much grander cultural level as proposed to just personal, so ittķwould be mych more unjust.Just imagine the whole populace of Japan literally working themselves to death ťñþso some lazy British chavs can lazily half-arse their way through life.