'email may never come back on-premises' ?
Outside the US, and with regard to the very long arms of the Patriot Act, there are many regulated market sectors whose data-security risk assessment keeps them well away from any US-domiciled service providers... and then there is the EU, with its new General Data Protection Regulation, which also constrains many sectors to keep the majority of their data [and data flows] well away from the major cloud service providers.
Many applications not handling personal data -are- suited to migration to cloud services;  many are already hosted somewhere by service providers hurriedly re-branding their hosted service as a Cloud Offering... but until there is clear legal separation between in-country operators and their US parent brands that offers true protection from data access by DHS, acceptable to regulators, many markets will ignore the benefits of cloud.