Building a digital bank for farmers in Africa. Was all text-based (USSD) and for those without phones we used NFC key fobs. Several million DAUs within a year.
In my opinion, there will always be a need for large companies to bring in outside help to get very specific things done (largely due to their size and dysfunction). MBB are smart enough to generally move where help is wanted. Just in my time there I saw them hugely lean in to big data, design, cloud, AI etc. And when they invest they go big.
That being said I personally would never bring in MBB for a purely technical implementation project. Way too expensive and the value is not there right now.
Will the big3 be around in 15-20 years or is it a dying industry with many of the best people leaving for exist opps in tech/finance/etc.