It will not affect numbers now, but it will in the future.
This is slowly corroding the middle of the pack mythic raids. As they suffer and stop raiding, people leave the game, but those are very small numbers for sure. Most of people dont even set foot on Mythic. But with this, in the long run high-end guilds lose material of recruitment, and their need of players will further cause problems for the middle of the pack guilds. You have less guilds for the high-end guilds to recruit from, so the middle of the pack loses more of its better players on a percentual scale. With this people on low Mythic guilds that want to step up their game and try to raid more seriously will have less guilds to go, but will be chased after by those guilds, causing problems for low mythic guilds to hold their best players. In the end the entire "food-chain" of guilds will suffer, and the gap between the high-end guilds and the common folk will increase, and the "path of progress" will be so convoluted that people will not feel like they can even try to progress.
Low Mythic guilds will suffer because they will never improve, because theyr best players will leave for middle of the pack mythic guilds, those mythic guilds will not sustain enough numbers to improve because they will be scarce and their best players will be taken by high-end guilds...
This happens since Vanilla, and EVERY time blizzard made a move that caused a big loss for middle of the pack raiders the game suffered huge losses.
Every time they did it the game shrinked badly. And it will happen again with personal loot.
Will it be the end of raiding and wow? No.
Will it be another big loss for the game? Yes.
The guild "food-chain" needs a steady income of new players and improving players. Personal loot cause problems for middle of the pack guilds, and reduce the incentives for people to fixate on a guild.