Many of the Marines at Iwo Jima had been through hellish combat on island campaign after island campaign. They did not have the benifit that their modern counterparts do of overwelming airpower. The US did dominate the air but had nothing like modern day close air support. In many of the earlier campaigns a US victory was not certain. Their enemy the Japanese did not surrender but fought to the last man. The Marines lives depended on the small arms they carried, while air power or naval bombardment could be decisive, it was not as it is today call on demand pinpoint accurate and lethally decisive. While Modern Marines and soldiers still depend on their small arms for survivial they are not near as dependent on them.
I seriously doubt than any group of US soldiers since Korea has seen the hell, the intensive fighting, and the long term close up grist mill combat that the Marines who fought in the Pacicfic endured and ultimately triumphed over.
Technically the M16 or M4 though it has a 30 round magazine does not have a really significant higher rate of practical fire than the M1 Garand. Side by side shoot offs sending 40 or more rounds down range with an M1 Garand and an M14 have been done before and the rate of fire was for all intents and purposes comparable. The Garand took an 8 round clip that could be slammed in without concern for up or down orientation and didn't have to be removed by hand or inserted with as much care or dexterity as a magazine. The modern M16 does not offer in the real world a substantially decisive advantage in practical firepower.
As far as individuals today being smarter or better than past generations - that is frankly laughable. Individuals today have the advantage of modern technology and are probably on average more technologically adept. However modern IQ's, or however you want to measure intelligence, are not any higher than that of our great great great great great great great great great great grandparents. Over the centuries mans technology has improved but the intelligence of the average individual has not changed appreciably, in fact there are some good arguments that if anything it has declined. In the past if you were a total idiot you died - today .gov takes care of you and yours.
The survival probablity of a modern Marine squad on Iwo Jima was probably less than the survival probabilty of a then contemporary Marine squad who would know the fight, the enemy tactics, and there own forces. They would also be familiar with the enemies weapons and radios and be able to utilize them effectively. The only significant advantage a modern squad of Marines would have is their night vision and possibly their body armour. Given the overwelming odds and firepower against them both would in all likely hood be wiped out to a man.
I'm not disrespecting our moderm Marines or soldiers, but squad level firepower is not a quantum leap ahead of WWII weaponry. Their firearms may have been heavier, but their level of effectiveness is generally comparable - one is not talking about comparing flintlocks to cartridge arms.
As has been referrenced before in this thread where the modern soldier beats the WWII soldier hands down in effectiveness and firepower is through the power of communication and combined forces - the coordination of artillery, airpower, mobility, smart weapons, and night vision.