I like this move for San Diego, assuming the money is reasonable. Johnson didn't get many snaps last season but graded out well, particularly in the slot, so he should be an upgrade on Eddie Royal. He is only 28, and Rivers will be the best QB he has played with by far in his NFL career to date.
This also means the Chargers now have Allen, Floyd, Johnson, Inman, and Jones at WR, so they don't need to spend a draft pick on a WR, which is good since they have so many holes to fill.
inman had a good game last year but come on. Floyd has some fits and spurts in him but what really are you hoping for from him? Jones is a special teamer who plays receiver. stevie and allen might be ok but there's no question the chargers could still use an upgrade. Not saying it's their only priority but it has to be on the list.
My view:
1. Allen will be back, hopefully healthier, and hopefully closer to 2013 form than 2014 form.
2. Floyd will likely be back.* Floyd has been quite underrated for years, he is much better than his numbers indicate, unless you look at advanced metrics. He is also a veteran with long running synergy with Rivers.
3. Johnson is an upgrade over Royal.
4. Inman's playing time was limited last year, but he flashed late, and I think there is solid potential there. It seems reasonable that he may be improved with a year of NFL experience.
5. Jones should be the #5 and hopefully won't see many WR snaps. But worst case, he is a wash with last year's #5, Ajirotutu. More likely, he is at least a marginal upgrade, if only because he is a better deep threat.
6. As someone else pointed out, technically Ladarius Green could (and many would argue should) take some WR snaps since he is languishing behind Gates.
I don't see the Chargers carrying 6 WRs on the active roster, and they just signed Johnson and Jones, so they are safe, and Allen is obviously also safe. Given Inman's potential and low salary, he would seem likely to be safe as well. So if the Chargers drafted a WR, in order for that WR to make the roster, it would likely mean releasing Floyd. I gave Floyd an asterisk, because his cap number is fairly high -- $4.7M. That makes this at least a possible scenario. However, I expect he will make the final roster, partly because he is actually a better WR than most realize, as I noted above.
The other reason is because the Chargers roster is absolutely riddled with holes. They still need to add OL despite resigning Dunlap and signing Franklin. They desperately need a quality NT, and they could also use more DL depth beyond that. They have lost their top 2 OLBs from last season (Freeney and Jarrett Johnson), ILB Gachkar just signed with Dallas, and they released LB Walker; meanwhile, Butler has been a huge bust. So they need LB help. They resigned Flowers and signed Miami DB Wilson, but they are still down two DBs from last year's roster and need more help there. They only have Oliver, Donald Brown, and Woodhead at RB, so they need another player there.
Finally, like some other great QBs, Rivers has proven that he can get it done with no name WRs over the years.
Given all that, IMO there is no way they should consider spending a draft pick on WR after signing Johnson, unless an incredible value falls to them.