Question:
"Do I have to upgrade my MBP to High Sierra?"
Answer:
No.
"I really do not want to upgrade."
Then, don't. It's YOUR choice.
Personal opinon follows:
I see no need to have "the latest and greatest", when what I have is working well for me.
I find the all the fear and loathing about the latest so-called "security flaws" to be... well... much ado about nothing.
Three weeks ago, these "flaws" didn't exist.
Then, suddenly, they were "discovered" by some kind of "researchers".
But, in reality, what has actually -changed- from three weeks ago, to today?
The CPUs that were in place then, are still there.
Everything that was working three weeks ago, is still working.
Who has actually been affected or suffered damage from such flaws?
When such events actually -happen-, then I may worry about them.
Not yet.
Not now.
My opinion only.
Others will disagree.
Some may disagree vehemently.