Prince William is reportedly “coming after” the carriages, costumes, music and theater of the monarchy he will one day lead, a royal insider spilled. This led royal expert Richard Eden — who spoke to a friend of the future king recently — to ominously reveal: “what he told me left me worried about the future of the monarchy.” Eden shared his conversation with the Prince of Wales’ pal while examining the “diminished” Buckingham Palace balcony royal appearances at Trooping the Colour since Queen Elizabeth II died. “When the older members of the family retire, His Royal Highness won’t be inviting…