>However, I am a strong believer in multi-tenant architectures as they allow to scale while mutualizing the resources (I personally think it's single tenant at scale is non-sense in term of ecology) and we will invest into maintaining a multi-tenant architecture.

Our products use multi-tenant architecture in the form of 1 database file per company, and a single database server for everyone (by default). It's great for data isolation, as we can't accidentally leak sensitive corporate data from one company's account to another (say, a missing WHERE). It's also great for indexing, as DB queries only touch small subsets of data. And it works well for most businesses (10-100 employees). For large companies (not that many of them), if we detect a lot of activity which stresses the main database server, we have infrastructure in place to migrate them to dedicated servers, transparently to users. It's worked pretty well so far.