I agree. At the same time, I don't think that you need to attack someone to point out what you're doing or what you're doing differently or how you set yourself apart from someone else.
Also, I think there's a difference between critique and attack and an idea/vision and a record of past actions. Buttigieg may not like Warren's and/or Sanders' ideas and vision and he may envision something else. But I'd rather he just tell me what he envisions than attacking someone else for what they envision. It seems unnecessarily negative.
In this case, I think this is generating some needed debate, since thankfully AOC and others have spoken up. I wish that Warren herself would more forcefully take on this one because it is at the crux of what the difference is between progressives and centrists.
The centrist Washington Post editorialists have been plying this ground for a number of months with criticisms of the proposals of Warren and Sanders. The idea is that if a program is not means-tested it is subsidizing the rich; social programs should be more modeled on charity of giving to those in need.
AOC's response is that only universal programs have the support of everyone necessary for them to be robust. This was the New Deal Democrats argument as to why we should have social security that covers everyone and later the same for Medicare. As AOC argues we don't exclude the well off from public schools, parks and libraries because they have the resources to provide their own.
Bernie makes these arguments in his campaign speeches but he doesn't get a lot of coverage for it. We need more discussion like this so that voters can hear the progressive argument rather than just the centrist argument that "we can't afford" to provide programs for all, we can only allow for charity-like and trickle-down policies which maintain the power of the wealthy economic elite.