Pitches weren't flat in the 70s, 80s, 90s, arguement = FALSE
Boundries were allot bigger then the ones today, arguement = FALSE (Australian boundaries are still bigger then any ground in any era)
Bats have always been noticiably bigger in the era, arguement = FALSE (Only since 2004-2005 and both Ricky Ponting and Matthew Hayden averaged well over 50)
Pace Bowling decline arguement = FALSE (Wasim Akram, Waqar Younis, Shaun Pollock, Allan Donald, Courtney Walsh, Shane Bond, Chaminda Vaas, Mahkaya Ntini, Darren Gough, Mohammad Asif, Dale Steyn + spinners: Muttiah Muralidaran, Saqlain Mushtaq, Anil Kumble, Harbhajan Singh, Shahid Afridi, Daniel Vettori)
Matthew Hayden > Gordon Greenidge or Desmond Haynes (How anyone can arguement against this is beyhond me. Hayden looks a far better batsman on footage and has way better statistics)
Glenn McGrath > Malcom Marshall (Considering that the people that think it's easier for batsman nowadays and was harder back then for batsman then logically it was easier for bowlers back then but harder for bowlers nowdays. It's funny because that is their logic but they still think Malcom Marshall is better then Glenn McGrath so ideally, they contradict their reasons that the 70s & 80s batsman are better then the batsman of the modern age.)
Points 2) and 3) are wrong. In the modern era we have boundary ropes (from 2000ish, cause I remember an Ind-Aus test in the 90s with no ropes), and the bats are HUGE. They compared the bat someone like Bradman and Pollock would have used to a modern bat on TV once, HUGE difference. Bradman used a stick, Pollock's was big by the standards back then but even I use a bat bigger than that now. So ropes = shorter boundaries, and larger bats = easier hitting.
And there has been a pace bowling decline. Compare the names in your list to Lillee, Snow, Hadlee, Thompson, Dev, Botham, Imran Khan, Bob Willis, Fred Trueman, Sarfraz Nawaz, Chris Old, Proctor, Alan Davidson. Pollock, Donald, Waqar, Wasim, Bond, Walsh. The others don't compare. Also, I've left out the names of all the West Indies quicks, which should add another 4 or 5 to the list. Not an easy time for batsmen.
As for spinners, Benaud, Bedi, Underwood, Qadir, Chandrasekhar, Iqbal Qasim, Prasanna, Venkatraghavan and Titmus shold do okay. Not as great as the Warne-Murali-Kumble-Vettori foursome, but still very good. Again, I've left out the West Indian spinners (Sobers, Gibbs, Ramdhin and Valentine).
As for the flat pitches argument, it is debatable, but it is a fact that Cricket viewership was much higher from about 1995 than it had ever been, and that TV broadcasters and cricket boards wanted 5 days of cricket. This has been the era of the super flat pitches, especially in the subcontinent. Places like West Indies and Aus//Eng/RSA on occasion haven't been much better.
Hayden does not look better. Greenidge and Hayne were beautiful to watch, I saw a few clips of them and understood why they were great. Hayden is sheer muscle and power. Someone like Hayden or Sehwag or Symonds or Flintoff is great to watch, but not as classy as Greenidge and Haynes. Plus, they face better bowlers on better pitches, and as a pair were lethal. They worked so well together.
Marshall and McGrath is very close. I've seen a little of Marshall bowling, but he swung, cut, seamed the ball all over the place on any pitch. McGrath is just super accurate with seam movement and bounce, and has done well of flat pitches. Dunnow how Marshall would have been on flat pitches (I think he did okay on the ones he played on) but he certainly seems more lethal to face. It's like comparing Kumble and Bill O'Reily. One was lethal, the other very effective. And while Kumble's achievements are incredible, many would preffer O'Reily as he could tear a lineup to shreds on his day. Bradman said he was the best bowler he ever face, but Kumble has the third most wickets in cricket's history and a 10-fer. It's a tough comparison, but many will go for Marshall because he was brilliant, regardess of the stats.