No, it's not impossible. It is possible if they completely take Schultz and Swaim (if he's back) out of the gameplan and at least 1 WR gets hurt.
There are like 20 "ifs" that would have to happen for a TE to catch 70 balls next year. And even then, catching 70 doesn't mean being an 800 yard TE. More likely for Jarwin is that 70 catches means just south of 600 yards, b/c the Cowboys use TEs as checkdowns.
Even at the end of his career Witten was doing better than the 8.6 yards per catch you say Jarwin would get, plus, for several years with Garrett as head coach Witten was over 11 yards per catch, so it's incorrect to say the Cowboys will only use TE's as a checkdown. Jarwin was at 11.4 yards per catch last season, and there is no reason to believe they won't try to get the same kind of production from him next year, but with more opportunities.
I do, however, agree that it's unlikely Jarwin will get 70 receptions, although he may get close. My guess is we will see around 85-90 total receptions from the TEs next season, with Jarwin to get about 65-70% of those, and Schultz and whoever else combining on the other 30-35%, Yardage could be anywhere from 800-1,000 yards. Of course, this is assuming we don't bring in a significant free agent, or draft a TE very early.