Every SaaS product starts with an architecture that fits the problem at the time. The trouble is, the problem keeps changing, with more tenants, more data, and more integrations, and the architecture that got you to your first ten customers is not always the one that gets you to your first thousand.
1. Tenant isolation is handled in application code, not the data layer
If deciding which tenant a row belongs to is a WHERE clause someone has to remember to add on every query, it is a matter of time before someone forgets. Tenant isolation belongs as close to the data as possible.
2. One noisy customer can slow down everyone else
Shared resources without limits mean a single tenant traffic spike becomes everyone incident. If you cannot point to how your system prevents this today, it is worth a look.
3. Every new feature needs a schema migration across every tenant
Some multi-tenant systems isolate schemas per tenant, which feels safe early on but becomes an operational burden once you are past a few dozen tenants and every migration has to run many times over.
4. Reporting queries compete with production traffic
If analytics and dashboards run against the same database handling live requests, growth in one directly degrades the other.
5. Nobody is confident about the blast radius of a bad deploy
In a healthy multi-tenant architecture, a bad deploy is contained. If a single bug could plausibly affect every customer at once, that is worth fixing before it is tested in production.
None of this means rewriting everything at once. Most of these are addressed incrementally, and the important part is knowing which one is costing you the most today.