I m in the same seat as you.
Holding on to my 2011 MBP and elaborating if I upgrade to the 2018 or wait for the 2019, or even wait further to the 2020 MBP.
My thoughts why I m looking now for the 2018 are.
1. They fixed all major issues (keyboard, heating and dual core processor)
2. Resell value will be very good
3. The 2018 MBP can be purchased with a nice discount in several markets
In my opinion, the 2019 MBP :
1. The next processor "Sunny Cove" will most likely improve GPU (estimated 50% increase) and partially CPU (Though not too much due to battery life constraints)
2. RAM Upgrade from DDR3 to DDR4 - which will have a negative impact on battery life due to higher consumption
3. Butterfly keyboard will stay the same - due to the fix introduced in the 2018 MBP version
4. No 4k display because of too much battery consumption.
So in general, only marginal improvements.
2020 / 2021 MBP will be an complete new cicle
It will be the first iteration of a new cycle. Looking back at all previous first and second iterations (2016 & 2018 MBP) a lot of flaws had to be improved (keyboard, processor, GPU) which where not detected during the development phase.
Which means you get a "feedback MBP" version which Apple will improve on.
That means for me, a reliable new MBP product for the total new cycle will be in 2022 or later.
I believe a 2018 will cover for the time until a next reliable version is available.
What are your thoughts on that ?