At 15W, that would mean charging your phone would take over two hours. That's way behind their competition, and twice the time of even their anemic 20W wall charger.

Even my friend's entry level $150 Xiaomi Note 7 comes with a 17W charger and supports up to 27W chargers.

Higher wattage degrades battery life, should only be used when needed.

Not a concern. His phone battery is cheap and easily replaceable.

He's been using it for a year and a half and battery is still fine. A bit over full day per charge according to him.