For one there are major balance issues with BGs that heavily favor certain classes(mw gaurdian rogues). If you think these balance changes will be addressed u are coping
Ontop of unbalanced team compositions that could auto decide who wins a match, you have less agency on the outcome of the match when you are 1/8 players instead of 1/3. Chances of getting trolled/griefed are now doubled.
Ontop of that 15 man base maps are compeltely garbage for 8v8 and incentivize people to avoid fights.