These tools are not bad
To be clear: for a personal blog or a first store with ten products, WordPress and Shopify are rational choices. Quick to launch, inexpensive, extensively documented. If that is your need, do not pay anyone to build bespoke.
We are talking about something else: sites and products that carry a professional business over several years.
The real cost of a theme
A theme is designed to serve thousands of different sites. It therefore ships hundreds of features of which you will use a handful, but which the browser downloads anyway. This is the most common cause of slow sites, and speed is a ranking factor at Google.
Then there is the plugin chain. Every plugin is third-party code with access to your database, which has to be updated, and whose update can break another plugin. You are no longer maintaining a site: you are maintaining an assembly nobody has a complete view of.
The real issue: who owns your business
With a hosted platform, your commercial terms depend on a third party. Pricing changes, commissions apply to your sales, and a feature can disappear between versions. For a business whose revenue runs through the site, that is a risk often taken without being measured.
What bespoke code changes
You get exactly the features you need, and no others. The repository is yours: if we disappear, another developer picks the project up. There is no commission on sales and no vendor subscription. And when your business has a particular rule — a per-customer price, a quote, an internal approval — it gets written, instead of worked around.
When bespoke is the wrong choice
If your total budget is under €5,000, bespoke makes no sense: take a turnkey solution and invest the difference in your content and your photography. We will tell you that plainly rather than sell you a project that does not add up.