I rarely post and don't think I've posted on this Season 4 thread in any case, but here's my take on the whole thing FWIW.
In my opinion, the Lagina brothers undertook this search in an attempt to prove or dispel once and for all the rumor of treasure having been buried on Oak Island. Like any other major treasure legend, Oak Island has its fair share of theories, adherents and opponents. Unlike most other legends, Oak Island also has a specific site which is not overly large and therefore is capable of being explored pretty thoroughly -- it's not the Superstitions!
Their goal, I think, is to be as thorough as they can be, given the time-frames they have to work within, the Canadian government red-tape and restrictions, and budgetary limits. By filming all or most of their work on the Island, the public at large can see how much is actually being done.
If they find treasure -- great! The public will see and know that there is no longer any reason for anyone to search further. If they don't -- bummer in that sense, but again the public will see and realize that it's unlikely there IS anything there, because the extent of the searching done (depths dug, areas metal detected, etc.) by the Laginas, in addition to previous large-scale searches done in the past, pretty much then rules out any treasure still in place if indeed it ever was there.
Either way, their search will essentially put to bed the Oak Island treasure mystery. A potential benefit is that the Island properties may then become open to sale for residential development or to some other use that has value to the property owners and which will be better-received if "treasure hunters" are no longer showing up. Dan Blankenship is not getting any younger, nor in fact is his son ... if there is after all no treasure to be found, it may be in their best interests to sell off their property to a condo development and retire to other climes.
Along the way, the Laginas are giving respectful attention to the most popular, as well as some lesser-known, theories about what treasure might be there and how it got there. Whether or not they personally adhere to the theories, they have looked deeper into some of them so that viewers can form opinions on how likely or unlikely they may be ... and if treasure IS found, then the research into these theories may be of value to the find.
I've followed all 4 seasons, and will look forward to a Season 5. The equipment being used and how it all works is interesting to me, as well as the various finds and what they are able to learn about them.