It all depends on what you mean by 5G
If you mean 3GPP release 15 - which is the official definition - then you can do all that at existing spectrum and not build anything. Go on marketing, call it 5G.
If you want Ericsson's vision of 5G which is 500MHz of contiguous spectrum then you are going to need to go to millimeter wave. That's lots and lots more sites. Maybe not towers but buying streetlights is still shareholder baiting capex.
If you want the IoT view of 5G which is a million connections per square kilometer you need lots of backhaul.Again not towers but significant capex.
And testing 5G with its MIMO, Beamforming and full duplex is hard, very, very hard much more so than 4G so again needs a lot more capex.
Thinking in towers is simplistic.