Still doing my research on it, but so far my understanding is:

The Lightning network solved some issues as a stepping-stone for scalability, but it didn't solve others, for instance privacy (made it a little better, yes, but there are still glaring issues).

Cashu is another stepping-stone, but a significant one in that anyone who joins Nostr today automatically has a cashu address without needing an account on any lightning infrastructure, and can redeem sats - No friction.

Further, the way ecash is minted is blind to pretty much anything that could identify a user or their funds, it holds no record of such things.

ecash tokens can be transacted offline (with a degree of trust still) due to being bearer assets, they are the money.

Because tokens can be transacted outside lightning before being claimed/redeemed/cashed out, it means zero fees. I think this is where the notion of "ecash" really hits.

In other words, it does custodial Lightning an order of magnitude better than current Lightning custodians.

If all the above checks out, I foresee Lightning being the place for infrastructure custodians and self-hosters, and cashu mints (and potentially fedimints) as the new wave of custodians. Previous custodians (like the Albys and the WoS' etc) will likely adapt, or be phased out over time.