IMO, the fundamental problems with Stallmanism are not to be found in its tactics and execution, but rather in the very core of the philosophy itself.
I completely disagree. There are some problems with the free software movement but the fundamental idea behind it is essentially inarguable: proprietary software is anti-user.
Thomas Schelling mentions the notion of "incomplete antagonism": you could have partly opposed interests to someone else's but still have ways in which you can cooperate with each other. Other people thinking about trade have suggested that this is almost always the case, even, maybe surprisingly, when people have directly opposed values in some respects (but not others).
http://www.amirrorclear.net/files/moral-trade.pdf
When you're buying most things from other people, you might have an "incomplete antagonism" because you may each want to get the best deal you can at the expense of the other person -- driving a hard bargain, so to speak. But you might still appreciate the opportunity to trade and not necessarily resent the other party to the trade (I wish that the vegan sandwich I just bought at LAX had cost me $8 instead of $19, but I'm still grateful that I could buy it at all).
Most attempts to defend proprietary software that aren't based in "romantic authorship" (emphasizing a special moral relationship between creators and their creative works that would make their preferences or interests count in a way that other people's don't) would probably focus on the benefits of trade: proprietary software fails to optimize perfectly for the user's interests and includes some intentional disadvantages for the user, yet many users could willingly accept these limitations because they've been bundled with benefits that the users appreciate, or because they help to incentivize activities the users appreciate (like continued development or support of the software).
This is normally the case for situations where trade can occur: each party may be asked to accept things that aren't that party's most preferred outcome, even deliberately where an alternative that that party prefers is logically conceivable. For example, if you rent a physical object then you might not be able to use it for any purpose, or if you hire someone then they might not be willing to perform any task you ask them to, or if you buy something then you might have to pay for it -- even though if others were making your interests their first priority, none of these limitations would exist. Presumably proprietary software advocates think that "selling software" is mostly akin to selling other things, where private ownership and sales work (fairly well|extremely well|better than any available alternative) for allocating resources, incentivizing productive activities, and increasing people's welfare, and that it's only anti-user in the ways that the costs or restrictions accompanying other kinds of trades are anti-customer.