Tennies aside for health reasons, an Angelican priest conducting an hour long service in Saturday-chores clothes seems akin to a judge tossing his black robe as meaningless.
They can wear dresses and ankle socks Monday to Saturday for all I care, but for one hour a week, maybe 3 at the most, there should be at least some semblance and reminder of dignity and respect when addressing and acting in the name of The Almighty.
If the vestments become more important than the Gospel, they may have a point.
But dumping vestments seems to me to be decreasing the impact of the Gospel in most places. Instead of treating Christ as important, it’s... something else.