Not everyone agrees that the freedom to start a SaaS business with GPL software is a "loophole".
Not everyone agrees that the freedom to make a SaaS business with a private GPL fork is a loophole or bug. It seems (from their creation of the AGPL and use of terms like "ASP loophole") that the GPL people do.
It's the perfect license for MongoDB and all these other companies trying to keep Amazon from taking their software and proprietarizing it... but instead they make weirdly restrictive one-off licenses nobody are going to contribute to.
Proprietary code is actually the most restrictive to both users and developers. That's because you can't legally mix proprietary code without a license at every step of the way. Any significantly large proprietary project becomes a mass of third-party dependencies with all sorts of license restrictions that your company is probably ignoring and breaking. Even if you follow all the tenants of those licenses, you don't own your code at the end. A Node-style mess of dependencies becomes ten times worse when each dependency has license restrictions attached to it.
In other forms of media, you generally don't see people try to license 30-50 different properties all at once for the same work, because that means nobody owns anything and the work will be really hard to market internationally. Software almost demands this sort of dependency soup, however, which creates all sorts of unique preservation problems. Whenever a large software project (or game engine) is discontinued, a lot of people wonder why the company who wrote it doesn't just release the code as Free for third parties to maintain. The answer is that in most cases, it's not their code to release.
Depends on what open source philosophy you support - do you believe users should have the right to look at your source code, modify it, and share the modifications with others? In that case only an open source license like the GPL, that protects the USERS right meets this requirement.
Every developer is also a user of their product. Thus, protecting the rights of the USER, automatically protects the rights of the developer too, to further study, develop and customise their code further. On the other hand, permissive license can restrict developers, as others can close-source your open-source work.
If all you want is recognition and credit for your work, then permissive open source licenses are definitely better.
What copyleft licenses prohibit is distributing software with less freedom than what you had when you received it, whether you modified it or not. But that's not a developer's issue, that's an issue for someone that hopes to gain something from restricting freedom.
Just because your core business software is AGPLv3 doesn't mean any other business will be able to build a coherent business model atop it, look at Kazoo by 2600Hz or Iris by Lowes.
Knowing how to properly configure and maintain said code in production, creating integrations with third party vendors (that your competition likely can't interact with on the same terms) and marketing to build mindshare among potential customers is a common way to offer your core software as AGPLv3 yet have no competitors or few competitors use your core software.
Neither GPLv3 nor GPLv2 prohibit this.
You're thinking of the AGPL, the first version of which was based on the GPLv2 and released in 2002, years before the FSF announced that they were even beginning work on the GPLv3.
AGPL does have restrictions, and it's anyone's preference as to whether they're good or bad, but there has never been an intent to forbid businesses. If anything it's the recent RedisLabs' modules license and the SSPL-style licences that do it: they explicitely contains the wording to indicate that the product can't be used to create a similar product and offer it as a service. Heck, the SSPL says (https://www.mongodb.com/licensing/server-side-public-license):
"If you make the functionality of the Program or a modified version available to third parties as a service, you must make the Service Source Code available via network download to everyone at no charge, under the terms of this License"
The AGPL tells you that if a user asks for the source code, you must give it to them and them only. The SSPL says that the source code must be publicly available to all, at no cost. It was created specifically to prevent competitors to run MongoDB as a Service. Surely if they had to change it's because the previous license did authorize competitors to do it. Wanna take a guess what the previous license was ?
The AGPL doesn't prohibit starting a business, it makes starting a business prohibitive.
Look at all the SaaS offerings of MongoDB (formerly AGPLv3): https://news.ycombinator.com/item?id=19362919
AGPLv3 doesn't make starting a business prohibitive, arguably forks like NextCloud would not exist if AGPLv3 did not compel codebase users to publish the additional code they wrote to extend OwnCloud