Yes, I believe that having all of the organization's tests in a single class is not a good idea. Some of the disadvantages that I perceive are:
Large classes enhance dependency, which means that several developers are more likely to work on the same class, increasing complexity.
It's more difficult to properly "scope" your tests when all the test methods are included in one large class. It is easier to understand what is being tested and to extend the class coherently when an independent class has test methods for a specific class.
Because only one method per class can be marked with the @testSetup annotation (https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex
testing testsetup using.htm), you can make greater use of the @testSetup annotation by using distinct classes.
These are some of the reasons (among many others) why you should separate your test methods into classes.