Simple solutions that work and are robust is good engineering.
Do not hesitate to suggest a better alternative. And then let it pass all analysis, use cases, reliability, regulatory compliance. We'll see how far you get. Probably thousands of engineers have already tried.
If the simple solution has the drawback of a critical device being way more vulnerable to external influence from common household items and appliances, then it certainly isn't a robust solution. I'd also doubt thousands of engineers have worked on this particular mechanism on pacemakers. It certainly is a legacy feature from a time where there really might've been no more reliable remote input mechanism than a reed switch so nobody ever dared changing that.
Deactivating the devices, in particular for defibrillators (likely a lot more than for pacemakers), must be easy enough and you may have to do it in emergency situations.
Accidentally (and temporarily) disabling an ICD is also not nearly as critical as suddenly changing a pacemaker to asynchronous mode. One of both can make you require a defibrillator real quick if you're unlucky. I don't see how random people/first responders would benefit from triggering magnet mode on a patient with such an implanted device.
"Oh, he's screaming in pain, I haven't even recorded an ECG, I don't even know how to read an ECG (is that the thing with the wiggly lines?), but it must be his ICD malfunctioning! Quick, take off your magnetic belt buckle and let me fiddle it over his chest in the hope of being somewhere near the implant's location!" - Someone, somewhere, maybe.
I have heard a lot of things, but that implantable devices are designed with lazy engineering, this is a first. So yeah, a bit of humility could help here. "There may be a better way, if we work hard enough on it, after considering ALL constraints and use cases." is probably more realistic than your statement above.
I was speaking in relative terms. Not using that better way that requires a lot of thought and hard work is "lazy". It's not like a corporation like Medtronic is on a tight budget, so there's no need to get overly protective.