A lot depends on how much money one is willing to commit to the system. As I said earlier, pro photographers will often elect to get the best, whether on land or underwater - it is their business, after all.
For the rest of us, it's usually a trade off between "how much do you want?" and "how much can you afford to lose to a flood?". As I said, I simply cannot afford to put something like a Canon 5DMk4 underwater, no matter how good the housing. The camera is actually more than I can afford for land use.
What I have found works is patience. When the 7DMk2 was announced, there was a flurry of activity from divers shedding 7DMk1 systems as they moved up. Some of these were incredible deals. In fact, that is how I got my 7DMk1 & housing.
I notice there are a few 5DMk2 systems for sale right now, so if that's up your alley then now is a good time to jump. Likewise a few 6DMk1 systems for sale.
The nice thing about getting an older system is the cameras are also coming down in price. I was able to buy a second 7D body for a very reasonable price.
I totally agree with having a second camera for the underwater system. Even if the housing floods, you can send it back to the manufacturer for a rebuild, but if you lose the (discontinued) camera, then the housing is a door stop. At least with a backup camera you can continue using the housing until you decide to upgrade.