If you have any savvy picking mutual funds, you should be able to get at least 10% per year long term. Yes, there will be ups and downs, but that's not an unreasonable average at all over the life of a DVC contract.
You had mentioned "investing" money to use for your Disney resort stay. So for an "average" deluxe resort stay of $2500 - $3000 for a week, you would have to invest at least $30,000 and get that 10% return. Oops, sorry kids, we only got 6% this year, so we have to stay in a Moderate or a Value. For us, that is simply NOT acceptable. We have not and will not ever stay in a Value resort, and only in a Moderate if we had no other choice, but that would be a 3rd or even 4th trip in one year, not for a single trip. Oh, and that is another thing. Your investment pretty much allows only 1 deluxe trip per year. We go at least 2 and up to 4 times every year. To get 3 trips for us, we would then have to "invest" $90,000, and still get the 10% return in order to afford 3 deluxe resort trips, and we would still not be able to afford a 3 bedroom Grand Villa in any one year. But we can easily get 3 trips out of our points every year, and our dues are under $2,000/year.
To your other point, when you buy most things you have immediate use of them. If I buy a luxury car I can use it starting on Day 1. The "invest the rest" is a valid comparison for DVC because your costs are frontloaded for a benefit that it'll take decades to realize.
ETA: And the "invest the rest" point is not about saving money for retirement or whatever. In my hypothetical scenario, you'd still use that money on Disney vacations.
When you buy your points, you have immediate use of them as well. We went on our first trip only 3 months after our purchase. The breakeven point at where you have saved enough on your resort costs to offset the initial investment is NOT decades (well, unless you'er OK with Moderates and Values - then DVC isn't for you anyway). We have already done that, so DVC is only costing us our dues each year. Granted, your breakeven point if you buy into VGF or Poly at $165/point will be longer - probably 15-18 years - but you then still have almost 35 years of dues only to save thousands every year. And the deed can be willed to your children for their use.
OK, so again, let's look at a real world example. Your kids are in school, so you have to go in the summer. You choose a week in July, and you want to stay at the GF every year. For the week of July 11 - July 18, your room cost for a lagoon view room this year will be approx. $3,153 without taxes. You claim you want to pay cash every year, and Disney for arguments sake, raises resort room prices every year for the next 30 years. To stay the same week on DVC points, you will need to buy 169 points. It will cost you a buy-in of $27,885 - there are no closing costs buying through Disney. Your dues first year will be $932.88. I'll break it down.
Again, assuming a 4% increase in both the rate of the resort room, and your DVC dues.
After 10 years, you will have spent $37,855 for your cash room. You will have spent $39,085 for your DVC buy-in and dues. You spent $1,230 MORE on DVC.
After 15 years, you will have spent $63,134 for your cash room. You will have spent $46,564 for your DVC buy-in and dues. You have now SAVED $16,569 with DVC.
After 18 years, you will have spent $80,859 for your cash room. You will have spent $51,908 for your DVC buy-in and dues. You have now SAVED $29,050 with DVC.
You have reached your breakeven point here.
After 20 years, you will have spent $93,890 for your cash room. You will have spent $55,664 for your DVC buy-in and dues. You have now SAVED $38,225 with DVC.
After 25 years, you will have spent $131,309 for your cash room. You will have spent $66,735 for your DVC buy-in and dues. You have now SAVED $66,735 with DVC.
After 30 years, you will have spent $176,835 for your cash room. You will have spent $77,296 for your DVC buy-in and dues. You have now SAVED $99,539 with DVC.
As a matter of fact, from year 20 to 25, and 25 to 30, you have saved more in each of those 5 years than your initial investment, and will continue to do so. I will never understand WHY you don't think this is a good deal for those of us that are only willing to stay deluxe, and visit WDW multiple times every year. I don't get it.