If you truly believe that DRM is harmful in general, you need to construct an argument that appeals to educating people about which practical freedoms they care about will be lost. At that point you need to accept that they might not care about those freedoms.
But if you construct an argument from the axiom that users shouldn't introduce anything into their systems they can't reasonably control, you've fundamentally isolated almost everyone who you want to convince. Of those remaining to listen, most will typically already be on your side end evangelizing the same thing. In order to be consistent, arguing from a core axiom of user freedom and choice requires sacrificing things like hardware security modules, which are a giant leap forward for user security. You can have the absolute freedom to do anything if you'd like, but you might find that there are relatively few ways to apply that freedom if you do.
The alternative is a balanced approach: you don't start from an axiom that users need to be inherently free, and that some amount of tradeoff in freedom is beneficial and desirable. Then you go on to demonstrate why DRM in particular is bad in general. But this is a much weaker position in general, and framed this way you'll have fewer people to argue with.
So make one. There'll be a lot of things it won't be able to do, but nobody's going to put a gun to your head and order you to stop producing a DRM-free machine for yourself.
Meanwhile, the "you don't control your machine" argument suffers from the same fundamental tension as many of the RMS/GNU/FSF positions: in the name of freeing you from restrictions, they impose restrictions. The GPL, in the name of protecting what you can do with the software, specifies some things you must never be allowed to do with the software. The hardcore anti-DRM position, in the name of protecting what you can do with the hardware, specifies some things you must never be allowed to do with the hardware.
Now, that's not necessarily bad; there are arguments to be made that trading off a small amount of freedom to do a handful of things protects a large amount of freedom to do many things. But unfortunately that's not a type of argument RMS/GNU/FSF can admit to, because they frame things in absolutist moral terms. The absolutist cannot speak in terms of tradeoffs. To do so is to compromise the entire position and, worse, to open the door to others who might propose a different set of tradeoffs and argue that they're as reasonable as the absolutist's proposal; at that point, as the famous line goes, you're just haggling over your price.
So if you want a computer with no DRM, by all means go and build one. Refuse to put anything in it that you don't absolutely control. You'll be giving up quite a few things, like being able to play back popular media, but if that's the tradeoff you want you certainly are free to go and have it.
What you can't do is force others to abide by that; most people simply do not care that there are things in their computers not fully under their control, or if they reflect on it are willing to accept the tradeoff of, say, having hardware/software that will refuse to play certain media unless certain third-party-imposed conditions are met. You could argue that the overwhelming majority of them have never thought about this and probably wouldn't come to a rigorous and well-informed decision about it, but morally you have no inherent right to substitute your judgment of what's good for them in place of their judgment of what's good for them. The most you can do is build your own computer which does things the way you want, install only software that you personally trust to abide by your principles, and advocate for others to do the same.