First and foremost would be financial transactions, especially banks, that if these links were lost might be unable to make settlements and reconcile daily balances with overseas subsidiaries, partners, and correspondents. That would have an immediate impact on any business that relies on making or receiving international payments to/from suppliers/customers - think SWIFT [0].

Secondly is the affect on 'cloud' services (AWS, Azure, Goggle) with their complex inter-dependencies across services and geographical regions. We've seen several instances over the last couple of years where even a minor failure in one system has a knock-on effect to tens of thousands of other organisations that rely on it. Also, consider the reliance that AWS globally appears to have on US-East. If AWS Europe data-centers loose their link to US-East there could be major disruption of many services hosted on AWS, even those that use multiple availability zones.

Third would be the knock-on effect of switching to other routes (BGP) and the consequent 'traffic jam' as traffic between North America and Europe attempts to use alternate routes - maybe via Latin America and Africa or Asia.

Fourth, telecommunications ('phone) and VoIP services linking North America and Europe, Africa, and Middle East.

Fifth, and possibly the hardest to mitigate, is the by-product due to extensive use of CDN 'edge' services (Cloudflare, etc.). Most web sites nowadays have many dependencies on externally hosted services. Many are behind CDN edge servers. If the origin servers for those are on the other side of a major break then suddenly services you'd expect to be fully local will break in unexpected ways.

[0] https://www.swift.com/