Certainly, developing our own pitchers, especially starters is a great way to build up a staff. We can see the boost Crawford, Houck and Bello have given this team and its future. We also acquired Pivetta, Wink and Whitlock, along with others, who have helps, but are not ace-types.
You set the bar at 1.5 years, and examples were given for 2 year guys being traded just months ago. Trades for proven, young or younger pitchers followed by extensions are numerous, as well. It is still a viable way to add a solid SP'er to a staff; it's just not been the Sox way since Nate (.5 yrs and re-signed,) Porcello (1 yr and extended 4 more,) and Sale (3 years and extended for 5 more.) Sometimes, trade & extensions come in below FA market value, which might be something JH would prefer over another David Price situation.
There are several ways to add pitching. DD and Bloom ended up providing a better staff than many of us imagined possible, of course with the help of Bailey and Brez. No doubt, our farm still looks pretty weak, when it comes to pitching, but we are strong enough and deep enough, there, to provide players and prospects to trade. Others have done it. It has gotten harder, as you have pointed out, but it is not impossible, and it is still happening, every winter.