Well, I don't claim to be a 'professional', my actual job is not photographic and compared to some of the oldies', I don't have the same years of experience; but I have owned more cameras than I should have and also have pondered this question. Since beginning my photographic endeavour I have owned a Canon 1DX, 2x 5D Mark III's, EOS M, 7D, Pentax K5, K30. Lots of L Series zooms and a handful of Pentax lenses. After taking a 'break' from photography, I am resuming my passion and continuing where I left of. I am buying new gear which will essentially be one camera one lens.
My weapon of choice will be [Hopefully] a Pentax K3 II and DA* 16-50 F2.8. Along with accessories such as an L Bracket and a bunch of GoPro stuff; this is what I am going to use for virtually everything. [I may buy a DA* 60-250 or DA* 50-135 for my remaining K50].
The choice is based on my own personal preference, obviously. But major factors are weather sealing [My poor cameras go to hell and back... sometimes just to hell
], GPS, overall versatility and image quality.
Some people will argue that you "Need" primes or only amateurs have one zoom lens. But, it is not equipment that takes amazing photographs [Something that cost me a lot of money to learn].
Also, who I personally believe to be the greatest photographer to ever live does most of his work on a single 24-70 f2.8 lens... Any guesses as to who?