That is true today if those 20 ppl are not coordinated. It was not true in vanilla either if they were coordinated. There are many battles raging between Mezz and New Home today where smaller, better geared and organized groups are wracking havoc to much larger groups.
That is just your opinion. This should be MMO. I (and many others) would like to participate in battles between 100s if not 1000s of people.
The point is that you have option today to do what you want either in skirmishes or in BGs.
And if you hate sharding so much just play clasic.