Yes and no. McCellen did take out the first at Yorktown and as you know was within four miles of Richmond.
And the Yorktown line imposed a delay of a month; it would have been less if the Navy had been willing to run past and bombard it, but as it happened the simple fact of the Yorktown bastion existing reduced Union options.
Again, no defence system is perfect, but the Yorktown bastion was
worthwhile.
Yes there were Confederate fortifications at Petersburg but they were overwhelmed
They weren't; Petersburg was taken when the Confederates abandoned it.
with Sherman's Army cutting off the rail lines and the fall of Wilmington Harbor the defenders and the people if Richmond were doomed to eventual starvation.
What you're describing is a siege, which would have worked just as well if the defenders had had an old style stone castle. There is no conceivable fortification system that can hold out forever cut off from the world unless it's big enough to be self-sufficient in food, population and weapons production, but if the only thing you can do to defeat a fort system
is cut it off from the world it's actually a pretty successful one.
My main point is simply that conventional war's can only be won on the offensive.
So? It's a lot easier to go
on the offensive, and stay there, if you have a solid set of fortifications to defend your homeland. If we imagine the hypothetical situation where any approach to Richmond involves going through a permanent fort network more than fifty miles from the Confederate capital (i.e. across all the good approaches) which
must be fought through and which
will take a month, then that means that the Confederate army can keep fighting in the north for at least a week or two after the main Union army has invaded the South.
Without the necessity to use the Army of Northern Virginia as the main shield of Richmond, the Union has to have an army up north sufficient to defeat the AoNV
and an army going after Richmond sufficient to defeat the AoNV; it never managed this.
Let's say the Union has 150,000 men all told, and the Confederacy has 90,000 men, and that attacking a fort is three times harder than fighting in the open field.
The Confederacy has forts, and puts 30,000 men into the forts. That means they have 60,000 men left over.
The Union wants to attack the South; how many men do they use to make their offensive on Richmond?
If they use 90,000 men, then they've got just about enough to fight the forts - but up north there's 60,000 Confederates and 60,000 Union troops; it's an even fight. And if the Confederacy opts to fight in the south instead, it's an even fight in the open down there.
Now the Confederacy puts 40,000 men into the forts. How many men does the Union put into their offensive on Richmond?
They need at least 120,000 to fight the forts fairly, but that means they've lost the ability to effectively defend Washington...
Obviously this is a hypothetical, but it illustrates that defensive forts are an advantage for the defender because they allow force multiplication; a siege is always going to work, but it's a much
slower way of losing than an army winning a pitched battle and then marching straight into your capital city.