I call this the Modernisation Spiral. I drew it to try to explain in one picture the myriad of target options that need to be considered when talking about Application Modernisation. I am sure I am missing a few.
There is not one size that fits all and the more apps, the more alternatives. Of course if there is more than one modernisation target then the next challenge is integrating them and working out the operating model for the potentially diverse set of solutions.
Message is: Don't assume that one single platform will solve all problems by default; of course, the more target platforms, the more complex and the more expensive the integration. So it is a bit of a balancing act to get the best collection of targets at an affordable cost.
ps. the photography buffs among you will recognise it is loosely based on the Fibonacci spiral :-)