Whenever I've used HEX, it's not because of the luxurious surroundings of a '332' - it's because it's the quickest and most convenient option. When I've arrived at Heathrow on a flight from overseas, the last thing I've wanted to face is boarding a slower (and potentially more crowded) stopping service into Paddington.
Strategically, using the 387s was the most sensible option because it used a fleet with some slack available in it and saved the expense of a completely new depot for the 332s, given that Old Oak had to be flattened for HS2.
Whether HEX will prosper is more a question of people's flying habits post-Covid - the fact that the Stansted and Gatwick operations have continued for years with Electrostar stock suggests that its success is not down to retaining 332s.