Purpose of ads.txt
ads.txt merely confirms who is allowed to sell advertising on your site. So in your case you would just list Google.
I'm not sure why this restricts things for your site? ads.txt supports multiple vendors.
How does this prevent fraud? If you have any third party content - articles, blog posts, comments, etc then a third party could insert their own unauthorized advertising in your site.
Also a hacker might hack into your site. Okay if they had root access they could change your ads.txt so that it is ineffective. But if you are running a CMS then it is much more likely they would only hack the CMS to insert their own advertising. ads.txt would prevent that from working.