The real culprit is what you mentioned: lack of options to spend DPs on at higher levels. Expanded skills/talents/class-specific abilities/etc. If this issue is solved, I think the Dynamic DP issue ceases to be a problem.
Agreed.... If we can provide options to help consume some of those DPs at high levels the issue goes away and character development continues.
I tend to disagree. For a power-gamer (which I tend to be) the incentive of, in the long run i.e. ~level 10 or higher, improving your stats without any negative impact on the available DPs is just to high to not invest heavily in stats. When using the 550 points for stats option from the HARP core rules and then invest 20 DPs into stats every level until all stats are at a value of 91, then the number of DPs per level improves until at, around level 10, you reach 72 DPs. And from around level 10 on, through this increasing number of DPs, the character got back all DPs he pumped into stats. So, on the extreme sides, for a development of these first 10 levels, you have the choice of either letting your DPs stay at ~40 or so from level 1 on, having 440 DPs to spend for skills and talents (double DPs in level 1), or you can pump 20 DPs into stats every level, having spent ~440 DPs for skills and talents
plus 200 DPs for stats. For a power-gamer that's and easy choice (unless you plan to only play your characters until level 5 or so)...
Unfortunately though, this kind of character development leads to quite boring characters that usually have 91 in all stats and afterwards only have their stats raised by very small amounts, i.e. they all look more or less the same stat-wise. Therefore IMO the current approach for raising stats is broken, since it encourages such poor character development - at least it does so for those who like to get the most out of the system for their characters.
A few relatively easy ways to fix this come to my mind:
a) Fixed DPs: Stats then only get improved if it fits the character concept (character player style) or to improve the relevant skills (power-gamer style).
b) A much reduced allowed number of DPs that may be put into stats improvements, e.g. 5 points. This delays the "break-even", where you got all DPs pumped into stats back via improved DPs per level, until very late levels, so that, at least for non-epic campaigns, you won't ever hit this "DP break-even".
c) Allow stat imrovements only up to a certain value, based on the initial stat value. E.g. it might only be allowed to increase stats until they are 20 (or 10) points higher than at character creation. That would mean that a stat initially set to 50 could be raised up to 70, and a stat initially set to 80 could be raised until it reaches 100. The obvious downside is that it would be required to track the initial values.
d) A combination of b) and c).
e) Stats cannot be increased via DPs but you have a random chance of having it increase when reaching a new level. Requires some more dice rolling upon reaching a new level. And somehow it does not really feel HARP-like.
All these alternatives (and you could probably come up with even more variants) have their pros and cons. The IMO easiest approach is to set the stats to a fixed value. I must admit though, that I think this is a bit boring. I always enjoyed it when my Rolemaster characters reached another level and their stats increased and thereby also their total number of DPs. Thereby the character came a bit closer to those epic characters of the game world in terms of stats. And better stats also gave a slightly higher number of DPs. So Rolemaster uses variant e), but, since this IMO somehow does not fit for HARP, I personally would prefer variant b), which has smaller stats gains (which also I prefer) and a very much reduced incentive for power-gamers to abuse the system by increasing stats.
YMMV