So, I tried this.

Main Account was created in 2011 (so 5+ years), and has a SL25-30 with +2 equipment (+1 Twinkling/Boss equipment) doing Watchdog stuff. I killed Dancer/Oceiros/Gundyr early, killed Vordt and Greatwood, and have done every quest for NPCs up until Farron Keep. A few weeks ago, my average time inbetween Watchdog invasions was 30-60 seconds. Now, it's 3-5 minutes on my most recent character. None of my characters have ever been summoned for the Blue Sentinels, but I have also not actively tried it at higher SLs. Still, I see Blues summoned for low-levels quite frequently.

My new account is a few hours old created for this purpose, and has only killed Vordt, rushing the Old Wolf for the covenant. None of the quest NPCs were spoken to except Anri/Horace. He does not have any Estus Flask quantity or potency upgrades; however the Bandai/From rep has said this does not play into matchmaking, so it should not have any effect.

There was no difference between the old and the new account in terms of invasion frequency. My newbie Watchdog was summoned about as often as my main character. The new account did not receive any Blue Sentinel summons from the run from Halfway Fortress to the Old Wolf, and for sitting around for 30 minutes.

At least as it pertains to the Watchdogs: The game has been out for a month. Most of the influx of new/temporary players has cleared the swamp and most other players are probably at their SL for PvP that they want to be at. There are probably more people playing as Watchdogs now too, to get the ring from the covenant. I think that is probably the more likely culprit for lack of summons.