Being an amateur or a pro provides zero benefits. Assuming you have the skills needed to plug it in, everyone will enjoy or suffer the same ROI, there is no benefit of scale, nothing to compete on, there is nothing you can do to get a better profitability than anyone else with the same hardware (and ultimately, same electricity cost which will become an issue again down the line). The only variable that really matters is time. Get your gear early, and you might get rich. Get them late and you will burn money.
As for continually upgrading; it doesnt matter. If a new faster/cheaper ASIC product comes out, either it will have a positive ROI or it wont. Who owns it, or how many he owns is irrelevant, its the same for everyone.
Allow me to define my terms a little better:
Amateur is someone who buys a piece of mining equipment and sits on it watching its returns decline and his equipment become worthless.
Pro is someone who keeps upgrading his average mix of equipment to sustain a growth rate of his hashing power faster than an average "amateur" or the network as a whole ( which is pretty much the same thing )
Now, given the above definitions, is it not the case that the pro will eventually be taking a larger share of the "fixed" pie ?
Yes it's true that for any given equipment purchase there is no advantage ... but it's the growth rate of hash power that determines the outcome not a particular snapshot in time