The stability of mpesa is the most endearing thing about it. If you have 1000 today and the shilling slides against the dollar overnight tomorrow you will still be having that 1000. It's only in a case of say Zimbabwe where inflation would go upwards of 200% overnight that this is bad because what you could buy with 1000 today you can't buy with the same 1000 tomorrow. Mpesa though is still a digital currency. You can buy/sell using it without having to involve cash! If our hard nosed barbarians at safaricom made it easier for every business owner to get a till number, mama mboga, jamaa wa kinyozi etc etc, people would hardly need to carry cash. Imagine ukiingia matatu kuna till number, ukinunua mayai boilo Ako na till pia, mútura pia. No one would want to carry cash, what would be the essence.
Just because mpesa doesn't appreciate and isn't traded like say Bitcoin doesn't mean that it is not digital. If I was at safaricom head of "different frontier" I would be in talks with Bitcoin on how to interchange currency between the two platforms seamlessly. So if for example I have 200000Ksh I can buy bitcoins and sell them later and still the cash back to my mpesa.
In the long run, Bitcoins still need to be converted to hard currency, unless you are in some shady deals like drugs, after a while you will still need that cash back in Kenyan notes. So even though one is way superior in terms of the tech behind it, they are still both digital currencies