A biosynthetic pathway does not run at a fixed rate. Its end product, call it Z, binds to an early enzyme in the pathway, usually the first committed step, and inhibits it. This is feedback inhibition: the product of the pathway controls the rate at which the pathway makes more of itself. The logic is economical. When Z is plentiful, the cell has no reason to keep spending substrate and energy on making Z, so Z switches the pathway down. When Z is consumed and its concentration falls, the inhibition is relieved and the pathway speeds up again.
The inhibited enzyme is almost always an allosteric enzyme: it has a regulatory site distinct from the active site, and Z binds there rather than competing with substrate at the catalytic site. This is why the inhibition is rapid and reversible. It does not require new protein synthesis or degradation; it is a direct consequence of Z binding and unbinding.
Now remove that regulated enzyme. Two things are lost at once. The catalytic step is gone, so the pathway is physically interrupted. But the control node is also gone. If the interruption is bypassed, or if the enzyme is removed in a way that leaves the rest of the pathway intact, the pathway can no longer be throttled by Z. It will run at whatever rate the upstream supply of substrate permits, regardless of how much Z the cell already has. The pathway is uncoupled from the need it was built to serve.