Matt has been a silent character for ages - they could have just let him slip away without a ridiculous storyline (the AmEx Russia story was also incredible, in the unbelievable sense, rather than amazing.) He could either then stay silent (cf Usha since ages) or they could quietly introduce a new actor, which at least is easier on radio than TV - we notice, but it's not as weird as a whole new face as on TV shows. One thing I've always liked about TA is that when there are funerals, past characters are sometimes mentioned as being there, even if we don't hear them speak; this is much better than your average TV soap funeral, where even close family members are often not even mentioned, if they are not a current character - yet funerals are usually all about, "do you remember...? Is so-and-so here? I'd love to catch up," and so on.
I agree with the comment upthread that plot should not override character, because it just loses credibility. Of course, IRL, some people do act out of character, so it can be done once in a while, but not frequently.
Another thing I like about TA is that it's not full of murderers - the death count on most TV soaps is ridiculous, and while I know a handful of people with criminal records, it's not every household in the street; I'd say the incidence of criminal activity in TA is probably about right and mostly credible - it's not constant plotting and revenge and murder. And screaming arguments. I know some relationships do revolve round shouting at each other on a frequent basis, but it 's actually not the default way of dealing with disagreements IME, but a rare blow-up - in TV soaps, they are always shouting at one another, and it's very tiresome. I don't want the incidence of affairs to creep up, either - EE is almost at the point where any new relationship, whether legit or adulterous, is likely to be risking some level of incest, as they've all slept around with each other so much. I really don't want TA to get like that - and it would be out of character for most characters.
I might expect some shouting when Ian finds out, and it's going to happen with Rob and Helen at some point, and she knows it deep down, hence the appeasement around the DNA test.