Solids are not a "thing of the past".
It all depends on what you want out of your experience and your PH's philosophy on hunting buffalo.
I wanted to do everything, including "stopping the charge." This is where solids can be important on buffalo.
When charged, you want bullets to penetrate as far as possible in order to take out the CNS, break important bones, and damage anything possible along their path.
I only wanted my PH to fire, if necessary, to save lives.
I carried a mixture of expanding bullets and solids.
My PH carried only solids.
My PH wanted me to do this, if the situation was "typical" (if there is such a thing):
1st shot - Expanding (DGX) - By far, the most important!
2nd shot - Solid (DGS) - If there was no risk of a "pass though" injuring another buffalo. Just to poke another hole through him.
3rd shot - Expanding through the heart (assuming he is on the ground).
4th+++ - Solids - To stop charges.
To me, this is, perhaps, one of the real advantages of a DR.
I start with a expanding bullet in the right barrel (forward trigger), solid in the left barrel (aft trigger).
Reload - same
Shot sequence - same
Reload - all solids from then on...
If the situation changes during the scenario, just know which trigger to pull to send the correct medicine.
Practice
Practice
Practice
Know your DR like the back of your hand before you leave for Africa. If you don't completely know it well, or are not completely comfortable with it, just take a scoped bolt-action with good expanding bullets (.375+++), do your part, and rely on your PH, if things get too close-up, fast, and dangerous.
There is no shame in that.
There should be much more shame in shooting 20 rounds through a DR at a rifle range, being able to put all bullets into a pie pan at 30m, and thinking you are completely ready for everything a buffalo hunt may entail.