There is two contributors to growth: increase in population, and productivity gains. If tech adoption slows down and population slows down, we go back to the historical norm of no economic growth.

Productivity gains don't only come from technological progress. Accumulation of capital, such as infrastructure, education, access to healthcare etc, also increase productivity.

Claiming "access to healthcare" is capital is a novel idea. It's a social infrastructure. It doesn't directly lead to production, any more than lunch breaks do.

Capital is not simply "anything that I can tie to improving my work output".

If you view the humans actually producing stuff as human capital, which many economists have done, then keeping that (human) capital in decent enough form by allowing it to have access to decent enough healthcare is a logical step forward.

We could then go a step or two forward and posit that a sick populace means a sick consumer class means reduced demand for goods that generate growth, but those are just details.

Of course I agree with this, but without technological progress this "saturates" rapidly. Only long-term durable productivity growth per unit of labor comes from technology - i.e. better ways of doing stuff.

All healthcare beyond the village witch doctor was technological progress at one time.

A somewhat unfair characterization of the historical norm. Before the Italian Renaissance, history was marked by periods of growth interleaved with periods of decline, in many cases because a societal crisis caused a destructive regime to take hold. For example, the Mongol conquests installed an extractive regime that was soon replaced by the ultraconservative Ming dynasty, which resulted in China's progress stalling for some four centuries.