What does the cost-per-dive comparison look like...
If I’m getting OW certification because I want to poke around the ocean near my house, my 1st monthly local dive might cost $500 for OW training, and $1,500 for a full set of basic gear, and $10 for a fill, and $20 for gasoline and some fast food, costing $2,030 for the dive. That's a lot for a dive.
The 2nd dive is going to require another fill, and more gasoline and fast food, so maybe $30 dollars for that dive, but it really isn’t $30 for the dive. It’s that plus splitting the cost of the training and gear between the two dives. So the actual cost of this dive isn’t $30, it’s that plus half the amount of training and equipment. The cost of the second dive is actually a little over 1K.
The third dive costs the $30, plus 1/3 the cost of the training and the gear, so now it's in the hundreds of dollars for the dive. After several months, the cost of each dive will be changing only a few dollars at a time, edging ever downward but in narrowing amounts, but it’s still going to be more than the cost of gear rental.
If you’re local diving monthly, then after a couple of dozen dives each dive will work out to maybe twice the cost of rental gear. However, since it may be worth it to have gear you know and trust, that is sized and tweaked to your exact needs, and that is ready to go regardless of whether a dive shop is open, it may well be worth it to buy your own gear early on if you can afford it. If you're going to dive a lot.
If one redoes these numbers to count OW training, and then just an annual dive vacation, the splitting up of training per dive and then adding it in to the cost of each dive does not make a lot of difference in dive costs. The bulk of the cost of diving ends up being pretty close to the annual vacation cost divided by dives. And by that measure, renting probably makes a lot more sense than buying gear. The financial calculation is just whether you want to pay for the dive vacation.
If you're going to keep buying gear, and especially if you're going to go into technical diving, then the cost per dive will be horrendous, and you're paying it because you like an expensive hobby, and have more money than brains. Or you just have a lot and lots of money.