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.