If I had bought into a competitor's product line and then saw Ego's stuff and how superior their battery technology is, I might be worried that they would go back to the drawing board to be more competitive.
Without an ounce of hesitation I can say Ego is so far ahead with their battery technology that I can't see them needing to change anything any time soon.
I believe that your concern is valid, however it should have been obvious to you that when you bought equipment with proprietary power sources, that you were locking yourself into that company for the live of the product. I doubt that there is a sufficient volume to interest any third party in producing replacement batteries.
That said, it appears that EGO is making everything backward compatible as far as the batteries are concerned. At least for now. Plus there are a few third parties who are in the business of refurbishing Li-ion batteries, i.e. replacing the batteries cells in the case. Not sure if they have the capability to do the EGO batteries right now, but that may evolve as EGO products become more common. Plus the EGO batteries have a 3 year warranty for what that's worth.
A more important consideration for me is the lack of a viable maintenance and repair capability. Right now the only solutions are to contact EGO for advice and DIY or to use Home Depot. This board has numerous stories on the long wait times for HD service. HD appears to be interested only in selling, not servicing the EGO equipment as all they can do is send it off and wait many weeks for repair/replacement.
Although I am satisfied so far with my 21" SP mower (except for lack of a decent vacuuming capability which MAY be resolved with the promised "high lift" blade early next year), I am not going to purchase any more EGO equipment until I see am improvement in the repair capability. Everything will eventually break and need service.
Dewalt now has the 60v that they are using for larger power tools like a TableSaw and a MitreSaw that will require more torque power than a drill.
EGO has the 56v and it's batteries are also large enough to power a larger motor required for outdoor tasks like mowing and snow blowing. They are nice enough to use the same battery for the smaller motor items trimmers, leaf blowers and hedge trimmers as well as mid sized motors like their chainsaw.
Now look back to the historical drill. it took really a tripling of power from the 6v to 18v to get a large enough improvement to really be noticeable. Sure there were some smaller bumps along the way like the 7.2v drill the 14v drill.. but you notice these are order of magnitude higher jumps. If you did a jump like that with EGO you would need to be up at full mains power voltage (but in DC) and that just is not likely to happen. Where EGO can make moves forward is more than likely into better motor technology and efficiency.
So am I afraid that EGO will change standards in the next 3 years, No. But only time will tell.
Will Ego raise their prices over time; I doubt it as that would be to their own detriment. And their Chinese suppliers at some point will offer "3rd party" battery options. If there is a demand, the market will supply.
Here's a YouTube video about the "law of diffusion of innovation" (Jump to 11:00, the link didn't work correctly):