In a USB Power Delivery system, a charger never forces power a device does not ask for. The moment a connection is made, the charger and device run a digital negotiation; the phone effectively says 'I accept a maximum of 20 watts,' and the charger complies.
This is why using a 100W PD charger with a phone that supports only 20W is completely safe; the phone simply draws what it needs, nothing more is sent. The actual risk comes from uncertified or older-generation chargers that do not support the PD standard properly.
The real danger shows up with low-quality products that fail to implement the PD protocol correctly; some of these try to push a fixed voltage without properly negotiating with the device, which can lead to voltage mismatches and potential damage.
So the practical advice is this: do not shy away from buying a high-wattage PD charger, since it will still be useful for whatever phone or tablet you buy next. What matters is that the charger is genuinely PD-certified and handles the negotiation process correctly.
Charjett's PD-certified chargers run this negotiation fully in line with the standard, delivering a safe charging experience no matter which phone or tablet gets plugged in.