there are stark differences in the examples you list.
1. When NW bought republic by purchasing it's stock republic was a viable going concern not in bankruptcy and NW assumed ALL contractual liabilities of republic (including their union contracts and associated seniority protections) and republic's employees had valid career expectations even if NW had not purchased it. Furthermore, in terms of size (hubs) they were about equal, NW had NRT and MSP and republic had DTW and MEM but NW was very heavy on international and widebodies whereas republic had none.
2.When DL bought western by purchasing it's stock western was a viable going concern not in bankruptcy and DL assumed ALL contractual liabilities of western (including their union contracts and associated seniority protections; although eventually only pilots remained unionized while the others eventually ended up non-union) and western's employees had valid career expectations even if DL had not purchased it. Furthermore, in terms of size they were about equal. At that time DL had hubs in ATL and DFW while western had it's SLC hub and LAX was a large operation (maybe hub) for them. DL flew L-10-11s while western flew the DC-10s.
3. When DL bought NW by purchasing it's stock with it's own DL stock NW was a viable concern not in bankruptcy and DL assumed ALL contractual liabilities of NW (including their union contracts and associated seniority protections; pilots are now integrated and union representation for the agents, ramp, and F/As are yet to be ascertained but DL is still honoring, by law, the contracts of these former NW groups) and NW employees had valid career expectations even if DL had not purchased it. Furthermore, in terms of size, they were about equal. DL had JFK, ATL, CVG, SLC, Europe and a large presence in South America. NW had DTW, MSP, MEM, NRT and the Pacific. They were pretty close in terms of widebodies.
4. AA DID NOT buy TWA by purchasing it's stock and TWA was NOT a viable concern and WAS in bankruptcy and AA did NOT assume all of TWA's contractual liabilities (only those AA deemed fit and it did NOT assume their union contracts in their entirety) and TWA employees had NO career expectations whatsoever and if AA had not purchased the assets it desired the TWA people would have been out of work and on the street. Furthermore, in terms of size, they were NOT AT ALL equal. AA had DFW, ORD, MIA, SJU, LAX, NYC , the Caribbean and Latin America. TWA only had STL. AA had about 700 planes while TWA had about 200. AA had at the time a huge fleet of widebodies (close to or over 100), DC-10s, MD-11s, A-300s, and 767 while TWA had only around 9, that's right 9 767s.
And finally AA had a net worth of about $7.5 billion while TWA had a negative net worth and was bankrupt and out of cash which is why AA wasted a couple hundred million in two installments to keep the corpse alive through the Ch11/CH7 proceeding.
You view it in an operational sense while I and many other nAAtives view it in a financial and legal sense which is most appropriate because the TWAers needed AA but the nAAtives and AA certainly did not need TWA.
PS: I want to make another point. You said "'Merger' is the generic word for taking two companies and combining then into one." On 4/9/01 TWA (Trans World Airlines) existed as TWA INC. When the "transaction" closed on 4/10/01, AA took the assets it wanted and placed them in an AA subsidiary called TWA Airlines (TWAllc) and paid TWA Inc. cash for these assets. So legally what you have is American Airlines Inc. with its new subsidiary TWAllc and TWA Inc. which is now a shell company consisting of the cash received from AA, the assets that AA didn't want (some aircraft), the liabilities that AA didn't take, and it's certificate of incorporation. So, according to your own definition, the two companies never merged.TWA Inc. shell company disbursed the cash it had to it's creditors. In the NW/republic, DL/western, and DL/NW examples you give, there are no shell companies of the acquired because, unlike TWA Inc., they were in fact merged with their acquirer.