I have not liked the word "cardio" for a long time. What exactly is it supposed to mean when you describe an exercise as cardiovascular? Think about how absurd this sounds. My heart beats as I type this, therefore this message is cardiovascular exercise. Cardio is an artifact of living—just as breathing. As "cardio" or "cardiovascular exercise" are poorly descriptive terms normally used to imply endurance exercise of some form, it becomes necessary to examine whether the scientific evidence is in favor of endurance exercise improving the health of the heart and circulation system.
O'keefe, James H., et al. Potential adverse cardiovascular effects from excessive endurance exercise. Mayo Clinic Proceedings. Vol. 87. No. 6. Elsevier, 2012.
https://doi.org/10.1016/j.mayocp.2012.04.005
Even when endurance exercise is purported to have "good" effects, the results are entirely suspicious:
Steiner, Sabine, et al. Endurance training increases the number of endothelial progenitor cells in patients with cardiovascular risk and coronary artery disease. Atherosclerosis 181.2 (2005): 305-310.
https://doi.org/10.1016/j.atherosclerosis.2005.01.006
>Regular exercise training augments the number of circulating EPCs in patients with CVRF and CAD and is associated with improved vascular function and NO synthesis.
Winlaw, David S., et al. Increased nitric oxide production in heart failure. The Lancet 344.8919 (1994): 373-374.
https://doi.org/10.1016/S0140-6736(94)91403-6
McCann, S. M., et al. The nitric oxide hypothesis of aging. Experimental gerontology 33.7-8 (1998): 813-826.
https://www.ncbi.nlm.nih.gov/pubmed/9951625
Beckman, Joseph S., and Willem H. Koppenol. Nitric oxide, superoxide, and peroxynitrite: the good, the bad, and ugly. American Journal of Physiology-Cell Physiology 271.5 (1996): C1424-C1437.
https://doi.org/10.1152/ajpcell.1996.271.5.C1424
>The direct toxicity of nitric oxide is modest but is greatly enhanced by reacting with superoxide to form peroxynitrite (ONOO-). […] Peroxynitrite modifies tyrosine in proteins to create nitrotyrosines, leaving a footprint detectable in vivo. Nitration of structural proteins, including neurofilaments and actin, can disrupt filament assembly with major pathological consequences. Antibodies to nitrotyrosine have revealed nitration in human atherosclerosis, myocardial ischemia, septic and distressed lung, inflammatory bowel disease, and amyotrophic lateral sclerosis.
Christen, Stephan, et al. γ-Tocopherol traps mutagenic electrophiles such as NOx and complements α-tocopherol: physiological implications. Proceedings of the National Academy of Sciences 94.7 (1997): 3217-3222.
https://doi.org/10.1073/pnas.94.7.3217
There can be no doubt that what you train for is what your body most adapts to however. It's just that only a select few are interested in becoming the best endurance exercisers, and there is concern that chronic endurance exercise progressively degenerates the health over the long term. Most seem to engage in endurance exercise not for its own sake but because of the belief it will help them in another goal.
Strength training doesn't necessarily seem harmful for the weakest of participants:
Radzewitz, Andrea, et al. Exercise and muscle strength training and their effect on quality of life in patients with chronic heart failure. European journal of heart failure 4.5 (2002): 627-634.
https://doi.org/10.1016/S1388-9842(02)00090-9
And it can improve certain measures of cardiovascular health:
Melo, Ruth C., et al. High eccentric strength training reduces the heart rate variability in healthy older men. British journal of sports medicine (2007).
http://dx.doi.org/10.1136/bjsm.2007.035246
Hoff, Jan, Arne Gran, and Jan Helgerud. Maximal strength training improves aerobic endurance performance. Scandinavian journal of medicine & science in sports 12.5 (2002): 288-295.
https://doi.org/10.1034/j.1600-0838.2002.01140.x
Hu, M., et al. Effects of strength training on work capacity and parasympathetic heart rate modulation during exercise in physically inactive men. International journal of sports medicine 30.10 (2009): 719-724.
https://doi.org/10.1055/s-0029-1225329