Right now the players that have the Ambition to defend a certain system surely can not enjoy it with the current mechanics. I don't want to be able to counterattack an undermining attempt by fortifying. I want to beat the attacker out of his skin until he stops. Undermining a system is an act of aggression just like breaking into your house. Oh, there is someone robbing you? Well you can watch your goods disappear and bring them back from another location but you can not kick the robber out of your house.
If explorers want to explore in peace i can understand they chose Solo. If a trader has the ambition to Trade around the whole galaxy without being disturbed i can understand that too. But i am not ok with people attacking my systems, doing things that might take a negative impact on me if they succeed without having the proper options to hold them off.
You
have the 'proper' option, but you don't seem to want to recognise or utilise it. It's quite clear from the way PP has been designed that FD want/expect systems to reach stalemate, ie fortification and undermining cancelling each other. What you're asking for is an extra component; confrontation, which FD evidently don't want to force on anyone since they know how badly it would p*ss off a whoooole lot of people. However, there is apparently plenty of confrontation right there (if you can find it through the dodgy P2P and instancing issues and the sheer scale of the conflict). There's been a whole bunch of people in threadzilla saying exactly the same thing you are, so leave the fortification to people willing to do it, and go and find each other, have a big ol' messy furball, may the Elitest dude win.
As for the state of 'your' systems, the simplest way I can think of to put it is, it's not all about you. Quite honestly, I don't care if you understand why I or anyone else chooses to play in group/solo, it's not your concern any more than why you choose to play in open, or what music you listen to, or what's your favourite fast food is ours. You don't have to 'be okay with it' because you don't factor into the decision-making process at all. Those of us playing in group or solo generally aren't doing so to deprive you of anything, or to undermine your systems while cackling evilly because you can't stop us; we're not playing
against you, we're playing
for ourselves.
Despite its real and/or perceived issues, powerplay has been quite cleverly designed to simulate the application of force on a cosmic scale, allowing for almost everything while not actually forcing anyone into anything.