To implement Nextcloud to the same level as Sharepoint, big money would need to be spent. Remember this data can never be lost, and needs little to no downtime. Building your own cloud service is no cheap and easy option. Epically when its a bank. Even a custom solution built on opensource tech was used, they would be responsible to keep it updated. That in itself is a massive task.
There is a reason why Azure and AWS based cloud systems are pretty much a standard. There isn't a solid alt.
O365 Exchange keep you free from worrying about keeping your server up to date. O365 Teams allows for not only communication but collaboration. Teams integrates with SharePoint allowing for project sites. O365 Sharepoint & Onedrive allow for extremely flexible data storage and data shares. With all the permission options you'd ever need. You can move your AD Login's user folder to use your personal Onedrive account, a highly effective tool.
There just isn't good options to replace O365. Google's GSuite is a crappy alt for Admins that couldn't figure out 365's complex admin consoles.