MacBook Pro battery capacity dropped; replace it?

Batteries are a wear item, like the soles of your shoes and the tires on your car. Their expected lifetime depends on use (as reflected in Charge cycles) and also in the passage of time, used or not. After a while, you should expect to have to replace the battery. 

Battery life  also depends on two factors you may be able to control a bit better. Lithium polymer batteries deteriorate when they get too hot, and when they are subject to very deep discharge. These should be avoided when possible.

Despite using these batteries in millions of Apple devices, there is still a lot of of randomness in exactly which battery will fail when.

Apple uses the criterion that a battery that doesn't consistently hold 80 percent of its original charge capacity should be serviced, and likely replaced.

Unfortunately, Apple no longer offers battery service for MacBook Pro models as old as your 2015 model.

"MacBook Pro battery capacity dropped; replace it?: Hello, i have got a macbook pro 2015 13inch, and when i opened coconutbattery, It says 75% remaining of capacity. but some 5-4 days later, it suddenly Reduced to whoppin 66%. i dont know what is the cause of this, i just used it for some light gaming, light game engine works, and some music production in REAPER. i got surprised when i saw it was on 66%. and i tried to look for batteries online (e.g like Shopee), it was expensive. should i replace the battery of my macbook or just keep using it?"

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Thank you for the screenshot.

VERIFYING THE BATTERY HEALTH ON A MAC:

You are using a Third-Party app here; it is not build into the macOS' encoding. So, rely on your Mac to investigate and view the Battery Health drop, as it is more trustworthy. As for the reason of dropping, it is no possible to know why, nor when, the Battery Health decreased. In fact, it can even increase in percentage.

1. Click: Apple menu
2. Select: System Settings...
3. Click: Battery
4. Click: the encircled "i" on the far-right of Battery Health
5. View: the Percentage of the Maximum Capacity.

Your Mac's Age is More than a Decade Olde:

You'd Mac is extremely obsolete, and is incompatible for more-recent macOS versions. So, consider upgrading, if not done so, already. Apple has a reputation for providing macOS update for their device fore 7 years, and then stops doing so. I'll keep my 2018 Mac, as it has an Intel Processor, for troubleshooting purposes, and ease of Virtual Machine use.

Put it simply – Apple wants to keep making money off you, so they design batteries that don’t last long.