I’d hold the pick. Williams may end up beig the best on the roster but even that isn’t a given. If he is in fact the best on the roster, won’t be for long.
I agree with you that the offer isn't strong enough to act on. Pick 13 according to my list would be Anthony Miller, Christian Kirk, Michael Gallup or Courtland Sutton (if I am not picking Gesicki or Lamar Jackson there). While I only see these WR as having tier two type upside, that is still pretty valuable in my view and more valuable than than Williams. The 13th overall pick could make an impact to the roster this year, or if not at least be a year further in their development than the 2019 2nd round pick would be. When you consider the time value discount that 2nd round pick in 2019 is closer to 3rd round type value.
That said if Williams is the starting RB for Green Bay ( I am not sure about that as I do like Aaron Jones slightly more than Williams) they might not be looking to replace him (or Jones) that soon. I have talked about this a bit in the Williams/Jones threads last year.
Here is the draft history of the Packers at the RB position since 2000
2017 4 134 Jamaal Williams RB 22 2017 2017 0 0 0 7 16 7 153 556 4 25 262 2 BYU
2017 5 182 Aaron Jones RB 22 2017 2017 0 0 0 3 12 4 81 448 4 9 22 0 Texas-El Paso
2017 7 238 Devante Mays RB 23 2017 2017 0 0 0 0 8 0 4 1 0 3 0 0 Utah St.
2015 6 206 Aaron Ripkowski FB 22 2015 2017 0 0 1 2 47 10 39 163 2 17 103 1 Oklahoma
2013 2 61 Eddie Lacy RB 23 2013 2017 0 1 3 36 60 51 857 3614 23 107 947 6 Alabama
2013 4 125 Johnathan Franklin RB 23 2013 2013 0 0 0 1 11 0 19 107 1 4 30 0 UCLA
2011 3 96 Alex Green RB 23 2011 2013 0 0 0 5 29 4 149 510 0 21 139 0 Hawaii
2010 6 193 James Starks RB 24 2010 2016 0 0 0 27 76 13 618 2506 9 125 1017 6 Buffalo
2009 5 145 Quinn Johnson RB 22 2009 2013 0 0 1 0 44 13 4 5 0 13 100 0 LSU
2007 2 63 Brandon Jackson RB 21 2007 2012 0 0 1 16 54 16 355 1383 7 112 864 2 Nebraska
2007 7 228 DeShawn Wynn RB 23 2007 2010 0 0 0 3 23 4 64 332 5 14 122 0 Florida
2002 4 135 Najeh Davenport RB 23 2002 2008 0 0 0 18 73 3 394 1819 13 54 538 3 Miami (FL)
2000 7 252 Rondell Mealey RB 23 2001 2002 0 0 0 2 14 1 22 73 1 9 76 0 LSU
What the Packers did last season drafting Williams and Jones mirrors what they did in 2013 when they drafted Lacy and Franklin. They did not go back to the RB well until Lacys rookie contract expired. They didn't draft any RB in 2018 (despite it being a very good draft for the RB position) showing their commitment to this longer term planning (imo) and being satisfied with the players they added last season. The 2019 draft is not expected to be very strong at the RB position just another wrinkle supporting the possibility that the Packers do not select RB next season either.
If Williams/Jones/Mays are good enough in their view, they might not be looking to add RB until 2021 when these rookie contracts will be expired.
Green Bay also not known for adding players in free agency as much as other teams, although this perspective may have changed with the new GM now.
So even if Williams and Jones are not top level RB talents, the Packers may not be looking to upgrade the position for a couple more seasons yet, unless they really faceplant I suppose. Based on early returns from both players and them not drafting any RB this year, those RB may still have more staying power with the team than they would with other teams looking to upgrade that position more frequently than the history of the Packers has been.