You missed the whole point in that uncalled for attack on my education. No one here is talking about preventing all competition. Then you went on repeating an economic principle without understanding it fully. I know that because you consider it absolute while it's in fact not. In the real world, it is very rare to find an actually efficient market with perfect and instantaneous discovery where those simplistic economic laws apply correctly.
The general point is that it is generally possible to keep fair competition flowing between a smaller group of companies, as long as that group is large enough for its members' respective interests not to align completely.
All other things being equal, there can be only one cheapest pack of toilet paper in a given market, which immediately disproves your argument. After all, having 5000 toilet paper manufacturers all competing among themselves is certainly no guarantee of any improvement to the consumer for that particular criteria, because 1000, 500, 100 or even 2 would have sufficed barring collusion.
Now we can add many other qualities to toilet paper that make discerning customers keener to see past price when they're buying between competing suppliers. However, in mature markets with proven, stable demand, there comes a point where adding more actors does not bring value. Those additional entities are merely tapping into existing market value without providing marginal benefits and without forcing others to improve.
Do you believe the toilet paper industry is so ripe with innovation that its warrants as many competing manufacturers as possible, with as much competitive spirit among them as possible? Nope. In a supermarket, the pack of toilet paper that's put in shelves slightly above eye-level will be chosen way more often by consumers than other packs located a bit below. Companies do not compete on the quality of their products, they compete on the amount of money they pay for their products to be stacked the right way on the right shelves at the right location.
Back in the real world, across many industries, going from thousands of competing companies to a few hundreds is definitely not worse for the consumer.