This gets argued as a matter of principle. It is a matter of arithmetic, and the arithmetic changes depending on who edits the site and how strange your requirements are.
When a template is genuinely right
A brochure site with conventional requirements, a team that publishes regularly, and a budget that needs to go further. A good theme gets you live in weeks and the ecosystem solves common problems for free.
There is nothing embarrassing about this. Buying a solved problem is usually the correct commercial decision.
When it becomes the expensive option
- Performance. A theme ships every feature to every page. Getting the load time back afterwards is a project.
- The nearly problem. A plugin does eighty per cent of what you need. The remaining twenty per cent is fought over for years.
- Maintenance. Core and plugin updates arrive constantly. Skipping them is a security problem, applying them without staging is an outage.
- Drift. Five years of accumulated plugins becomes something nobody wants to touch, and eventually nobody can.
The three-year comparison
Compare total cost of ownership rather than build price. Take the build, add three years of maintenance, add the hours your team spends working around the thing that nearly works, and add the rebuild if the answer is a rebuild in year two.
On that basis a template is often cheaper for a simple brochure site and often more expensive for anything with real logic in it. Our view on both is under custom web development.
The decision in two questions
Who edits this, and how often? If the answer is weekly by a non-technical person, a CMS earns its overhead.
How unusual are the requirements? If any part of the workflow made you say "but we do it slightly differently", that part will not fit a plugin, and it is worth finding out now rather than in month four.