This isn't a new idea, but it is a good one. If you've ever read the Dune books, that's one of the themes. The most powerful noble house in Dune would take its soldiers from a prison-planet. The prison-planet was quite chaotic and brutal, so anyone who could survive and thrive on the planet would be quite capable. This idea worked... except the nomadic people eking out a living on the desert planet Arrakis were even tougher since their planet was even more inhospitable and dangerous than the prison-planet. They weren't bigger or meaner than the other warriors, but they were tougher and stronger from having to endure such an incredibly harsh environment from birth.

How do you create better weapons than the ones you have now? Chain a bunch of wild animals in a planet-sized pen, toss in a super-powerful magical artifact along with some of your best attempts at soldiers (Earthen, Mechagnoms, proto-Vrykul), set it to simmer for a few hundred thousand years, and any intelligent creature that can survive will be pretty tough.

The Old Gods know that they can't take Sargeras on toe-to-toe. They need soldiers who are resistant to his corruption. So, they tossed their best soldiers in the Old Gods' prison to see what would happen. Any creature to survive would be pretty resilient to corruption and capable of fighting forces of chaos. It's just the same way some disease vaccinations works. You get a very weak shot of a disease, which prompts your body to react and create antibodies. Now you can resist strong versions of the same disease. Your body knows how to react to it. The Titans clobbered the junk out of the Old Gods, rendering them quite harmless for a long time, allowing the beings on Azeroth to slowly get used to their influence.

The Old Gods provided two critical parts, not only the source of corruption to be able resist, but also the source of chaos and dynamism to cause the beings on Azeroth to be able to change to resist it (the curse of flesh).

I find Algalon's observations on feeling and loving life as telling here. We know what the Titans are good at creating; machines. Very clever machines, but static, unchanging, unadaptable machines. Machines can't feel or think or decide. They can't learn or desire to be better or push themselves harder. It could be that the Titans realized their commitment to supreme Order left them entirely incapable of fighting the teeming, formless Chaos that Sargeras controls. Keep in mind Sargeras is one Titan who apparently fought the entire Pantheon to a standstill. They needed machines plus some chaos... they needed something like the curse of flesh, which was beyond their ken.

The whole point is it resist their own urges to tinker and micromanage, and let a little chaos creep in and see if their soldiers could react/resist/adapt to it.