Edited out
There are already pure casinos out there that do not have a sportbook integrated and they will have a tougher time growing a loyal player base. I imagine that the most profitable online gambling sites have both, because players might like to spend their money on either. Sports fans will also be spurred back to a particular site if they like it, when there is a special or big event in a particular sport (like the Cheltenham festival for horse racing, the masters for golf or Wimbledon for tennis). You'll often find special promotions around these top events in the sporting calendar too. It might be profitable to run a casino without a sportbook but it'll be a lot harder and you better have something else unique to offer your customers.
Most of the casinos that doesn't have sport betting on their platforms already felt a big impact of that on their revenue generation and how their grow in the market, alot of them already go out of business and that speak alot of volume of what is happening currently in the gambling market industry.
Sometimes, I think that building a sportbooking sites is far more lucrative that building an entire casino which have other in house games, although when you take of games that generate more revenue for the casino then you definitely talking of in house games, but in terms of popularity, and wider market coverage, sport bets is more popularly acceptable to the general public and more friendly to newbies.