Well this will be his 2nd Pro Bowl and 2nd season leading the NFL in receptions. He currently has 441 career receptions, so he should finish around 450 for this season. He's 28 years old, so let's say he has 3 more years playing at this high high level he's at.
If he has 3 more years left in his prime, he could get close to 800 catches. From there, he'd need 200 more catches from ages 32 on to reach 1,000 for his career - a pretty big career milestone for receivers.
With that all said, I always lean towards players who were dominant for a smaller period of time over players who had very solid careers but over a longer period of time. I call this the "Ricky Watters vs. Terrell Davis Debate". If you look at Watters career numbers, they dwarf Davis's:
Watters (10 seasons): 10,643 rushing, 4,248 receiving, 91 total TD's
Davis (7 seasons): 7,607 rushing, 1,280 receiving, 76 total TD's
However, if you watched the NFL in the 1990's and especially the late 90's and were asked "Who's the best running back in football?" - most peoples answer would have been Terrell Davis, especially considering he had a 2,000 yard season. I wouldn't have considered Ricky Watters to be the best running back in the NFL at any point in his career.
So I traditionally give less credit to a guy who just "stuck around" and got to 10,000 career rushing yards over a guy who had legendary seasons albeit for a much shorter time.
So to tie this all back into Welker, I would lend myself to vote for a guy who had 450 catches over a 4 year span over a guy who had 450 catches over a 6 year span but more longevity.