FM2436-
The funny thing is, most of the city pairs from BWI you listed WEREN'T "high-density" before WN started service. Taking DOT's compilations of daily passenger counts on several routes:
ALB-BWI: Before WN (1Q00), 74 (pax/day); Most recent (2Q03), 621 (pax/day)
BUF-BWI: Before WN (1Q00), 94; Most recent (2Q03), 757
BDL-BWI: Before WN (2Q99), 241; Most recent (2Q03), 933
IND-BWI: Before WN (1Q00), 193; Most recent (2Q03), 476
ISP-BWI: Before WN (4Q98), 26; Most recent (2Q03), 542
MHT-BWI: Before WN (1Q98), 25; Most recent (2Q03), 1531
PVD-BWI: Before WN (2Q95), 159; Most recent (2Q03), 1715
ORF-BWI: Before WN (2Q01), 96; Most recent (2Q03), 216
RDU-BWI: Before WN (1Q99), 121; Most recent (2Q03), 575
PBI-BWI: Before WN (4Q00), 239; Most recent (2Q03), 439
JAX-BWI: Before WN (2Q96), 199; Most recent (2Q03), 602
MCO-BWI: Before WN (3Q95), 426; Most recent (2Q03), 1965
TPA-BWI: Before WN (3Q95), 354; Most recent (2Q03), 1298
I can't get figures for before 4Q93 which would give traffic numbers from BWI to STL, MDW, BNA, STL, CLE, HOU, and LAX before Southwest. But of the markets I listed, the smallest percentage increase was to PBI (84%) ranging up to a ridiculously large increase of 6000% in traffic between MHT and BWI.
I wouldn't exactly call PVD-PHL a high-density pair, either, at a whopping 116 daily passengers. But with the stimulative effect of the AVERAGE one-way fare dropping from $325 to under $50, it wouldn't surprise me to see traffic increase by 1000% or more on that route. Same for MHT-PHL, at 102 daily passengers paying an average one-way fare of $303. Most of the other routes announced by Southwest already have or will have non-stop low-fare competition (TZ to MDW, HP to PHX/LAS, FL to PBI/MCO/TPA/FLL, F9 to LAX).