The highest purchase in their history and the highest salary they've ever paid anyone since Zlatan/Eto'o. His pedigree - (Everton and WB, no disrespect to them) and the 2 seasons he had at Manchester - did not warrant that. It was an immense gamble on Inter's part to spend that kind of money on him.
The narrative that Lukaku alone made them a title-contender is ridiculous. Conte bought many players both starters and bench players (Barella, Hakimi, Eriksen, Bastoni, Perisic, Lukaku, Sensi, Vidal.. ). Most were within budget, Lukaku was not. Conte and Yang made them the best side in Italy, profiting from the decline of an aging Juventus managed by inexperienced Pirlo.
Chelsea buying him bailed them out financially, and replacing him with Dzeko and Correa doesn't seem to be a big loss goalwise. It even gives them more depth.