This is really a fallback-cost decision
Families often want to go directly to the main sight once they arrive. With seniors, the real issue is not the toilet itself but the cost of needing it after the group has already walked in or joined a queue.
Once that happens, the whole visit can start to fragment.
If the later path is uncertain, toilet first is usually cheaper
When toilet locations are unclear, walking is longer or return access is awkward, handling it first is usually the stronger move.
That choice protects the rest of the visit rather than wasting time.
Main sight first works only when the return is genuinely easy
If the toilet is clearly nearby, the main sight is short and the senior traveller feels stable, going to the sight first can be reasonable.
But this needs a concrete basis, not a vague hope that things will probably be fine.
The best outcome is an unhurried finish after the sight
A good decision leaves the senior traveller able to finish the sight, drink water, rest and walk back calmly.
If the group ends up rushing for a toilet or a seat immediately after the highlight, the order was probably wrong.