Players don't get to decide if they can or cannot make an ability check. Nobody's entitled to an ability check. It's for the DM to use when the outcome is uncertain.
I know your position on who decides when to roll, dude.
That's not what I said. I said the *character* is capable of making a check.
As in, the rules contain within them clear instructions on how being a a low dex dwarf in plate mail affects stealth checks.
If someone was on sentry duty, and such a dwarf was trying to sneak past them (he is obscured from sight) the rules are pretty clear: the DM can have him roll a Stealth check at disadvantage with a -1 penalty from dexterity.
So if your position is that when that dwarf is trying to sneak up on someone who is foraging he automatically fails... We have a weird disconnect. He's actually worse at stealth when the person he is sneaking up on is distracted?
Or, less weird but still weird, you decide that if it's a situation where you'd let him roll vs. a sentry then vs. a forager he just auto succeeds?
It avoids the most egregiously backwards scenario. But then I'm back to a previous comment that this removes any sense of granularity between various levels of situational awareness and various levels of stealth training. Everyone is the same in this situation, which, while arguably consistent with the RAW, makes no damn sense.