I know this is an anecdote, but I have quite a few 18650's from early 2010's still in service, of course with significant increase to internal resistance and drop to capacity, but fully functional otherwise.

I also have many functional 18650s of that age. lithium ion cells often self discharge at a few % per month, so after a few years of no power use a cell is likely to be dead without recharging anyways. The betavoltaics will still have usable power output. The real question is what the cost/performance comparison looks like between a betavoltaic system and a 18650 + tiny solar panel system that can on average recharge the battery on an annual basis.

He's talking decades without recharging.

Wonder how early EVs with 18650s like the tesla roadster (2008) or model s (2013) are faring?