I believe 18k is conservative because currently rETH TVL is 187k ETH. With Shapella in about 2 months and some continued growth until that point we can expect say 210k ETH by then. Completely ignoring solo staker migration or new node operators attracted by <16 ETH minipools, we need 35% of existing minipools to convert to support rETH TVL of 360k ETH, which would mean that 5% of TVL and appropriate DP size is 18k.
Using the same logic for 15k, this would be appropriate for no solo stakers, no new NOs and 21% of existing minipools converting.
10k limit would likely be below the 5% target just from growth until Shapella with 0% existing minipools converting. Keep in mind that this increase is proposed to take place 10 days before Shapella, so most likely at some point in March.
Maybe @ken can add a forum poll with these options to gauge community sentiment?