Why don't you look at this link. I think it will prove your understanding is incomplete, and that water does have significant absorption in the microwave region of the spectrum.
If you re-read my comment, you'll note that I only said that 2.4GHZ was in the 1-20 GHz band that is easily absorbed by water.
Engineers juggle multiple constraints, cost, size, power, and efficacy, and 2.4 GHZ choice was the result of the size of the magnetron that would be cost effective and physically fit into the oven. But it did, of course, have to be within the range of frequencies best absorbed by water, which 2.4GHz is, and which is why it's difficult to get bluetooth through your body.
There are regulatory limits on how much RF a device can send into the body. These are called SAR limits [1] and they are not changing. I was joking about the body effects. The legal limit is not changing. But devices that used to be further below the limit, will now be closer to it.
The higher data rates are hoped to at least partially compensate for the higher power. You burst data at high power, but for less time. Also, there are retransmits that occur now, if a packet is corrupted. Higher power should result in fewer retransmits, which will help with battery drain. Battery life is hoped to be better, even with the higher power.
Getting 2.4GHz through the human body is hard. The human body is mostly water, and bluetooth is, after all, the same frequency of your microwave oven, which uses 2.4GHz because it's in a band of frequencies from 1-20GHz that water absorbs.
When you're indoors, you don't need to get bluetooth through your body. You're getting reflections off of nearby walls and ceilings which allow your bluetooth devices to communicate across your body, but without going through your body.
When you're outdoors, you no longer enjoy the benefit of reflected RF, and the design of the phone and the headset antennas needs to be very good, so the RF can make it through your body.
It is a hard problem, but a lot of headset manufacturers do achieve it. I'm surprised you still haven't found one that's acceptable. Of course, it's body dependent. Petite women will have less issues then large men, as the RF just has less water to travel through with them.
Today's bluetooth is limited to 4dBm max transmit power for class 2. Bluetooth 5 will be 20dBm, which is a lot more power. This is, actually, the same power that class 1 bluetooth devices now have, so I'm unsure why they brag about the higher power of bluetooth 5, but to be sure, most bluetooth devices now are not class 1.
The higher power will make even bad antenna engineers be able to get bluetooth through your body, but more importantly, you'll enjoy larger range when you are at the gym or in your home. Also, if you worry about RF effects on the human body, your worry can increase now as well!
https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_...