Why do you want to convince them?
It’s really hard to change someone’s mind. We all know this, and we’ve all had the experience of getting stuck with someone who’s really defensive and refusing to listen to our increasingly impassioned arguments (or we’ve been on the receiving end of this - where we really dig in hard and don’t want to listen to the other person).
This is a well-known phenomenon that happens for a bunch of reasons, but one of them is that people are skeptical of each other’s agendas. And in particular, if you don’t know what someone’s agenda is, you will make one up - we’re all a bit paranoid, so usually we will find a “worst case”. And that leads to defensiveness.
One way to get past this is for you yourself to ask “why do I want to change this person’s mind?” and answer it as honestly as possible before engaging - and then be up front about it! In technical arguments, there’s often an aspect of politics or status or ego involved - the arguments are less about the tech and more about the value or status of the people involved.
So if you can ask yourself this question, and the answer is something like “I’m worried your design will be too costly in resources” or “I’m worried I know about some issues that I don’t think you’re aware of, that will impact the decision”, you often can start to find your way to a more collaborative and helpful posture. Someone will react much better to “I am worried you don’t know something and will waste time on this design” as opposed to them inferring an agenda like “I think you’re dumb and I’m trying to expose you and hurt you” (which people really do unconsciously think!)
If you can genuinely understand your motivations, and they’re genuinely meant to be helpful to the other person, expressing them is a great way to take some of the defensiveness out of the situation - now you’re both on the “same side” looking at the challenge together.