I have said that upgrading from a 12" to a 16" or from a 16" to a 20" was not that dramatic more than once, the difference is not worth an expensive upgrade, spending money for gas to drive to a dark site is a better idea.
I've owned some scopes for 50 years - including a 20-inch Dob for about 25 years (on my 2nd one F/3.5 vs F/5). I spend some nights at Bortle 2 - Cherry Springs, where I have an annual Galaxy Pass (not quite 3-hour drive) - and a 'camp' in the Adirondacks - side-benefit = Bortle 2 / SQM = 21.85.
I would agree that a person with a 16" Dob for a year or two should concentrate on just observing - and getting themselves and the Dob to Bortle 3 or better as often as possible.
Now let's assume - they already observe often under dark sky - they know 'the drill' with getting the most out of a big Dob - they know what they want to observe, and the 16 is not getting it - an upgrade to a 20 may well be in order.
Dramatic increase? That would be going from my friend's 16 or even my 20 to another friend's 36" F/4 with 3-inch ES 30mm 100 eyepiece -- or even Al Nagler's NV device. Now *that's* dramatic! 
Anyway - I certainly *can* see a difference in the views provided by an excellent 16 and an excellent 20 under Bortle 2 sky. For example - details in the spiral arms of M-51 or M-101 - more seen in the 20. A few months ago at Stellafane 2023 I had my 20 F/3.5, PII, ES 25mm 100 on M-13 and two galaxies - one 9th, one 16.5 magnitude - in one field of view I had - Milky Way field stars - maybe 50 to 1200 light years, M-13 a sea of stars at 22,000 light years, bright galaxy at maybe 30 million light years, dim 16th mag fuzzie at maybe 300 million light years (easy direct object - no averted anything needed to see it). None of the other scopes in the area could provide that view - 12 too small - friend's 20 F/5 with 20mm Nagler - field too small to see all three. Another time - friend with a 22" F/3.6 at Cherry Springs had a list of "billion light year galaxies" - we went hunting with his scope and my 20 F/5 -- while any one the 22 showed was seen in the 20 -- they were just a tad brighter in the 22.
As of late I'm liking Mel Bartels recommendation for building (and buying) 'the next Dob' in your yard -- decide on the focal length - then thro money and effort at it to get to the largest primary that will still 'work' (provide a 7mm or 6mm exit pupil - maybe an F/3.0?) - with of course the limitations of $$, ease of use, transport, etc. - but those things differ for each observer.
Bottom line - a very good 16" Dob will provide a lifetime of excellent observing for anyone -- but there will always be a few who want more and are willing to spend the $$ and deal with the 'lifestyle changes' needed to make use of it. I expect to see at least one 30", maybe three, on the field at Cherry Springs on any clear weekend night for a long time to come.