How I'm fixing the 401(k)
Guideline isn't actually "fixing" your 401(k).  They are primarily just competing on price.  Employers can already get a decent 401(k) from Vanguard, which obviously offers access to their low-cost funds.  Guideline doesn't appear to be offering anything fundamentally new (in contrast to, for example, robo-advisors).  If I were setting up a 401(k) plan for a company, I'd probably just go with Vanguard, with full confidence that my provider will not undergo any "growing pains" or even cease to exist in 5 years.