Yes extension to Mathura is perfect, will be popular too but with PM ke time nahi bachega aur Bikaner aur Jaipur Alwar Rfs ki nazar rahegi extension ke liye. So better to keep it upto Agra.
Mathura se veerangna special permanent nahi ho paayi aur pehle hi log extension ki demand karne lage.
Plus it will loose it's main aim of connecting 2 ends of bundelkhand