Jenkins, Adam Jones and Phillips all at their best were possible World XV (okay maybe Phillips is pushing it, but you can see the point I'm making) and now all appear finished as a force in international rugby. They might hang around longer, but they've are all a LONG way from their peak and are unlikely to rekindle their old form now.
Those are 3 key players to the side that was winning and are not easy to replace. Jenkins has been waning for over a year or so now and had several injuries take it out of him and is getting targeted by refs. Phillips has been waning ever since the World Cup in all honesty. Adam Jones is in a pickle with his contract mess and his career had now gone full circle, from subbed off at 30 minutes, to one of the best props in the world, to subbed off at 30 minutes again.
I can't emphasise how important the scrum was during those back to back 6 Nations ***les. Gatlandball at it's absolute best always came from the opponents being whistled off the park, like 30-3 and the Lions against Australia in 2013. Now Adam sadly is ageing and waning, Gethin Jenkins can no longer be carried and is getting pinged by refs. That England game in 2013 could never happen now with the scrum engagements now exposing the looseheads more. Likewise Hibbard, who acted like a 3rd prop before, is not so effective as a hooker. The front 5 in 2012/13 has gone from a strength, to now average (Ian Evans form has gone AWOL as well and Charteris is ever crocked). Only AWJ I could make an argument for now as a Lions starter (and argument for not certain starter in case there is confusion), a year ago the entire front 5 made the Lions squad and were in contention.
It would make sense for Paul James, Ken Owens and Samson Lee to be the first choice front row now. Assuming Gatland dropped his bizarre fascination with Rhodri Jones at tighthead.
And as an aside, that 2019 contract for Gatland is one of the most absurd things I've seen. It's overtaken Eddie O'Sullivan's bumper pre-2007 World Cup contract as one of the most ill fated I've seen. It's time for some fresh ideas in the coaching staff. Gatland will likely finish the year with a 1-28 record against SANZAR nations. SANZAR simply worked him out, now Lancaster and Schmidt have as well and he doesn't appear to have a Plan B or adaptation once sides work him out. Hence the constant losses to Australia.