The triangles point to a little bump in the groves near the centre of the tire. The grooves are gone (?) and the bump was gone a long time ago. You will probably see the wires soon and the fabric at the centre of the tire.
My Dunlop rear got replaced at 9,800 km with a Michelin City Grip and I eventually replaced the front at 21,000 km with a City Grip also.
I am currently trying a Michelin Power Pure on the rear. I think the City Grip was better but ask me again in about 10,000 km.