Most orchid culture is inert anyway, so there is no static resource pool in the old pot in the first place. Nutrients are introduced in the form of fertilizers, and unused portions are washed away when watering.
It will be no different in the new pot - except of course, fresh media, better air circulation, and more room to grow.
And roots are not the only means for nutrient intake. Leaves are also able to take in moisture and nutrients to a certain degree.
If the plant is repotted in such a way that a significant portion of the healthy roots and leaves are damaged, then the plant will be stressed out - with or without flowers. The flowers did not cause the plant's decline. The grower did.
The flowers do not draw nutrients from the plant. The plant sends it to the flowers. Big difference there in terms of who decides where the nutrients are going. The flower cannot force the plant to give it nutrients if the plant does not want to.
Orchids are experts in resource management. It knows if the plant can no longer sustain the flowers and will discontinue the blooms if necessary. It will also evacuate all available nutrients from old tissue and divert it to the new growth to give it a better chance.