What “senior engineers only” actually buys you
The expensive part of software was never the typing. It's knowing which thing not to build.
Agencies love a pyramid: a senior name on the pitch, juniors on the keyboard. It scales headcount and margin, and it quietly transfers the cost of inexperience onto your timeline. You pay senior rates for a senior to review work a senior could have done right the first time.
Judgment doesn't delegate
The hard part of building software is not writing the code. It's the hundred small decisions per day about what to build, what to skip, and what will hurt you in six months. Those decisions are judgment, and judgment is exactly the thing you can't hand to someone who hasn't been burned yet.
You can supervise effort. You cannot supervise taste.
Staffing senior-only is more expensive per hour and far cheaper per outcome. Fewer hours, fewer dead ends, fewer rewrites of the thing that was built before anyone understood the problem. The bill at the end is smaller, even though the rate on it is higher.
The trade we accept
It means we stay small. We can't take every project, and we can't grow by stacking juniors under a brand. That's a constraint we chose on purpose — it's the only way the rest of the model holds together.