A growth in the market cap is exactly what we need to see, as a higher market cap means that new money is being pumped into the market rather than more BTC being unlocked.
Au contraire. Current ATH mcap we see is exactly because more BTC has been unlocked/mined in the last few years, since BTC didn't reach ATH yet. Personally, I don't think that mcap is very important metric as it can be very misleading. It's nice to look at, but nothing else really.
First of all, it's often used by various shitcoins to attract new investors who decide whether to buy or not something depending on the mcap rank. For example, you can make some shitcoin with huge supply, get it on some irrelevant exchange, pump the price and get unrealistically high mcap.
2nd, as we all know mcap is calculated by multiplying current price and current amount of coins in circulation, but then again we often have no idea what's the circulation. In BTC case, 18,547,962 has been mined up until now, but are all of those coins in circulation? What about all lost BTC, should that be counted too?
In the end, if mcap increases or decreases by xyz billion, it doesn't mean that amount of money entered or exited that specific market.