Doors and Windows - Rotted Door Threshhold

We have been in our new home 2 years. All doors were special order. The threshhold to exterior door in master BR is rotting. The door is cracking at the bottom. I have seen this threshhold problem before. How can this be stopped PERMANENTLY??? We have gutters, but rain blows against this door.
Bob Vila had a special about this in the past. He used some sort of tar-looking paper under the threshhold and all these other steps, but I can't find the info.

There is a reason that roofs were placed over most doors in days gone by.
Some wood specie can stand up to the conditions to which your door has succumbed.
A white oak or teak threshold is capable of standing up to weather. However the existing jambs are going to rot, & the door itself will fall apart in quick fashion.

The reason that rot is so prolific on windows & doors of recent manufacture is that the quality of materials is so low. Couple that with mediocre assembly standards & the need to make a profit, & you will not find a door made that is able to withstand frequent water soaking.
A standard door made 100 years ago would fare much better than a new one, but even they were not impervious to rot. Oak sills were common, & redwood trim was standard on mid-priced homes. The old growth pine that was common had enough tannin to retard rot.

You could contact the door manufacturer & procure a replacement thresh. It will last the same 2 years as its predecessor if no preventive action is taken.
Treating all components with a wood preservative like Cuprinol (green #10) will prevent rot for many years. The green will bleed through paint for a while, & that is the trade-off.
The only way that I would do the repair is to completely remove the door frame, & install the new thresh to it while it is on sawhorses.

A storm door, as ugly as they are, will shed the water, but a roof is really the only practical solution to avoid constant problems with this door. You did ask for a permanent solution…………