Projects that start out as basic HTML/CSS (perhaps with server generation, like Rails) can run into major problems when they gradually need to start adding interactive features (which, in my experience, is almost certainly going to be the case with any company whose website is a significant part of their product and who have not yet found product-market fit).
It's really easy to use jQuery or some small purpose-built JavaScript library to add simple interactive features like a datepicker or accordion menu. But then you start building your own interactive features, with no framework or architecture in place to manage the ever-increasing amount of client state that is changing in a user session.
At some point (and I would argue it's pretty early on), it becomes far simpler to have a client-side UI framework/library/architecture in place, even if it is initially too heavy for your current needs. Your website is probably not going to need less interactivity as your product and business progresses.
It's really easy to use jQuery or some small purpose-built JavaScript library to add simple interactive features like a datepicker or accordion menu. But then you start building your own interactive features, with no framework or architecture in place to manage the ever-increasing amount of client state that is changing in a user session.
At some point (and I would argue it's pretty early on), it becomes far simpler to have a client-side UI framework/library/architecture in place, even if it is initially too heavy for your current needs. Your website is probably not going to need less interactivity as your product and business progresses.