Realistically, if we gave Ireland and Afghanistan Test status we'd end up with a less-formal, overlapping tier system anyway.A single tier doesn't work because nobody is going to tour Ireland and Namibia.
Australia/England/India/South Africa would play each other regularly
SL/Pakistan/WI/NZ would play each other regularly
Zimbabwe/Bangladesh/Ireland/Afghanistan would play each other regularly
And then you'd still have crossover series that would almost work as an informal promotion/relegation system. One of the big bois start losing a number of series, they go and play more against Pakistan/NZ more and the resurgent Sri Lankans play against South Africa more regularly.
It's more-or-less how it has developed now, and realistically it isn't the worst arrangement. Issues do arise from funding/finance though, especially given Zimbabwe's situation.