I certainly understand the theoretical advantages of the modern design but I question whether those advantages apply in at least one very common scenario. If this boat is 32 feet and used for very long distance cruising, ie a dinghy or two, 3 or 4 anchors and rodes, 40 days of food for 2, etc.etc, is it still going to be just as stable and fast as when it was test sailed by the owner? Personally, in my opinion, this modern design is going to prove to be slower and MUCH less stable than had it been designed a little heavier in the first place and with a little more boat in the water.
I completely agree with what you are saying if we compare boats based solely on length on deck. If the comparison is two equal length boats that are equally loaded, one a traditional long waterline relative to LOA, heavy L/D boat, whether full keel or some partially cut away keel form, and the other a more modern equal length boat, the newer boat will not be able carry as much weight in gear and consumables without seriously compromising performance and seaworthiness. But also, since design loads on the hull and rig are proportionate to the displacement of the boat, when equally loading these two boats a lighter modern design will be loaded proportionately more as compared to its design displacement and so would end up being a comparatively less robust boat when loaded to the max loading of the equal length heavy d/l cruiser.
It is for this reason that I have always suggested that a better way of comparing offshore boats is to compare boats of equal empty displacement rather than boats of equal length on deck or waterline. In that comparison, within reason, the longer boat of equal displacement will generally offer better performance across a wide windspeed range, offer better motion comfort and seaworthiness, be easier to handle, have better carrying capacity without impacting handling or seaworthiness and have roughly similar operating costs.
I once counted the rolls of 2 very different designs as they were sailing DW off Baja. One, a full keel 32, the other a modern 36. The full keel 32 rolled exactly 30% less than the 36. That is, the 36 rolled 100 times during the 32’s 70 times. Both boats rolled at what seemed the same angle. The 32 was sailing about 1/4k faster. Are future cruisers going to have to give up comfort for the modern design or go much longer in WL to get reasonable comfort?
Anecdotally I can imagine how this would be true more often than not. Similar to the discussion above, a heavier (D/L) displacement boat will typically offer a slower roll rate, which can be a very good thing in a comparative short chop or in long wave length ocean conditions.
Depending on the wave frequency, and the weight and bouyancy distribution of the two boats, the heavier boat may also roll through a narrower roll angle as well.
But if we compare boats of equal displacement, one longer than the other, the longer boat will generally offer a similar roll rate, but if the boat is a modern design which has been properly designed for motion, the roll angle should be smaller and the edges of the roll more gentle. But also the collisions with waves when beating and close reaching should also produce less hobby-horsing and a softer collision with each wave.
On the other hand, depending on the wave frequency, and relatve speeds of the boats, the longer boat for the same displacement may not fare so well with regards to heave. Typically a longer boat with a smaller D/L, when encountering a single large wave or a widely spaced wave train, will heave at closer to the speed of the wave than a boat with a heavier D/L giving the heavier boat a much more comfortable heave motion in those conditions.
On the other hand, the heavier D/L advantage can get lost in a repetitive, closer spaced wave train, where the inertia of the heavier boat can take it our of sync with the wave train and so cause it to hit harder at the trough and peak of the wave resulting in a less comfortable motion in a narrow range of conditions.