Your request/expectations of the Varia Vision goes well beyond it's intended purpose. But market pressures do drive changes but you'll have to convince Garmin with numbers.
When Garmin, or any business, contemplates a new market segment, they need to know how big it is and whether the sales they can produce will justify the cost of serving that market segment.
I doubt your market segment, “24 / 7 visual heart-rate monitoring” using the Varia Vision or something like it is big enough to justify developing for it.