You can go more than basic heart rate monitor since you can easily host a single lead ECG sensor with the "decade old" hardware that you are referring to, just like the latest smartwatch from Apple and Samsung. Heck, you can even host a multi-lead ECG sensor that is more capable than the latest smartwatch products from Apple and Samsung with the "decade old" processor. Couple that wearable smartwatch based multi-lead ECG sensor with any latest Android phone then you have a potent and energy efficient ECG system that is probably as good as the most expensive ECG machine at any hospital but powered only by a coin battery! This out-of-hospital gadget based on "decade old" processor is going to be useful to detect heart arrhythmia and ischemia that is the main cause of mortality in the world and capable of detecting pre-symptomatic Covid-19 as well [1].
I know this is a bad cliche, but this "decade old" processor is more powerful than the most powerful PDP-11 systems that have ever existed, that were last sold back in 1990s.